Question: What three chords does McCartney generally make use of?
About the article "Hey Jude", section "Composition and structure".

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In music, a major seventh chord is a seventh chord where the "third" note is a major third above the root, and the "seventh" note is a major seventh above the root (a fifth above the third note) .
The major seventh chord, sometimes also called a "Delta chord", can be written as maj7, M7, , ⑦ etc.
It can be represented by the integer notation {0, 4, 7, 11}.
In the case where the seventh note is a minor seventh above the root, it is instead called a dominant seventh chord.
Examples of tonic major seventh chords include Bread's "Make It With You", America's "Tin Man", Blood Sweat & Tears' "You've Made Me So Very Happy", third and main part of Paul McCartney and Wings' "Band On The Run", Carly Simon's "The Right Thing To Do", Rupert Holmes' Him and, most famously, Chicago's "Colour My World".
Moving in the seventies to replace the prominence of the dominant seventh as a stable tonic more common in the first fifteen years of the rock era, the major seventh was common in all styles, "pervading soul, country rock, soft rock, MOR (middle-of-the-road styles), jazz rock, funk, and disco."
"In soul and disco, a tonic minor seventh harmony often alternated with a dominant seventh or dominant ninth chord on formula_1 ['Lady Marmalade' & 'Le Freak']...
In other styles, major seventh and minor seventh chords generally mix (usually with eleventh chords...) to create a diatonic composite in either major or minor mode...
The most famous major seventh chord in the history of music, the one that opens...'Colour My World', even though the song departs from the usual pattern described above by 'colouring' the harmonic succession with several chromatic chords.
Still, seven of that song's fourteen chords, including the tonic, are major sevenths or ninths, demonstrating the primacy of that chord type.
"According to Forte, the major seventh chord is exemplified by IV7, which originates melodically.
The just major seventh chord is tuned in the ratios 8:10:12:15, as a just major chord is tuned 4:5:6 and a just major seventh is tuned 15:8.
Pieces which feature prominent major seventh chords include: Chick Corea's "Litha", Joe Henderson's "Inner Urge", John Lennon's "Imagine", Freddie Hubbard's "Little Sunflower", Carole King's "It's Too Late", Michel Legrand's "Watch What Happens", Antonio Jobim's "Dindi", Red Hot Chili Peppers' Under The Bridge, and Tadd Dameron's "Lady Bird". 
Stylistically, reggae incorporates some of the musical elements of rhythm and blues (R&B), jazz, mento, calypso, African, and Latin American music, as well as other genres.
Reggae is played in 4/4 time because the symmetrical rhythmic pattern does not lend itself to other time signatures such as 3/4 time.
One of the most easily recognizable elements is offbeat rhythms; staccato chords played by a guitar or piano (or both) on the offbeats of the measure, often referred to as the skank.
This rhythmic pattern accents the second and fourth beats in each bar and combines with the drum's emphasis on beat three to create a unique sense of phrasing.
The reggae offbeat can be counted so that it falls between each count as an "and" (example: 1 and 2 and 3 and 4 and etc.)
or counted as a half-time feel at twice the tempo so it falls on beats 2 and 4.
This is in contrast to the way most other popular genres focus on beat one, the "downbeat".
The tempo of reggae is usually slower than ska and rocksteady.
It is this slower tempo, the guitar/piano offbeats, the emphasis on the third beat, and the use of syncopated, melodic bass lines that differentiate reggae from other music, although other musical styles have incorporated some of these innovations.
Harmonically the music is essentially the same as any other modern popular genre with a tendency to make use of simple chord progressions.
Reggae sometimes uses the dominant chord in its minor form therefore never allowing a perfect cadence to be sounded; this lack of resolution between the tonic and the dominant imparts a sense of movement "without rest" and harmonic ambiguity.
Extended chords like the major 7th ("Waiting in Vain" by Bob Marley) and minor 7th are used though suspended chords or diminished chords are rare.
Minor keys are commonly used especially with the minor chord forms of the subdominant and dominant chord (for example in the key of G minor the progression may be played Gm - Dm - Gm - Dm - Cm - Dm - Cm - Dm).
A simple progression borrowed from rhythm 'n blues and soul music is the tonic chord followed by the minor supertonic chord with the two chords repeated continuously to form a complete verse ("Just My Imagination" by The Temptations C - Dm7).
The concept of "call and response" can be found throughout reggae music, in the vocals but also in the way parts are composed and arranged for each instrument.
The emphasis on the "third beat" of the bar also results in a different sense of musical phrasing, with bass lines and melody lines often emphasizing what might be considered "pick up notes" in other genres. 
With his writing techniques, Wilkins prefers to start with chord progressions as opposed to lyrics.
He has said he attempts to be a “vessel” in which music just flows out of him naturally.
He prefers writing and producing together to see the song all the way through both processes to make sure it’s a hit.
When writing Wilkins chooses not to use many sounds but only what is completely necessary.
He says he tends to write simple lyrics that people can grasp with a running theme throughout the song.
He likes to write simple chords to pop songs and focuses on the melody.
When he can Wilkins prefers co-writing.
Generally Wilkins focuses on the music and the melody and collaborates with lyricists. 
Adding a minor seventh to a major triad creates a "dominant" seventh (denoted V7).
In music theory, the "dominant seventh" described here is called a major-minor seventh, emphasizing the chord's construction rather than its usual function.
Dominant sevenths are often the dominant chords in three-chord progressions, in which they increase the tension with the tonic "already inherent in the dominant triad".
The dominant seventh discussed is the most commonly played seventh chord.
An A-major I-IV-V7 chord progression A-D-E7 was used by Paul McCartney in the song "3 Legs" on his album "Ram".</ref>These progressions with seventh chords arise in the harmonization of major scales in seventh chords.</ref> 
Understanding a particular musical style's treatment of dissonance — what is considered dissonant and what rules or procedures govern how dissonant intervals, chords, or notes are treated — is key in understanding that particular style.
For instance, harmony is generally governed by chords, which are collections of notes defined as tolerably consonant by the style.
(There is likely, however, to be a hierarchy of chords, with some considered more consonant and some more dissonant.)
Any note that does not fall within the prevailing harmony is considered dissonant.
A given style typically pays attention to how its musical structure approaches dissonance (in steps is less jarring, a leap is more jarring), and even more to how they resolve (almost always by step), to how they fit within the meter and rhythm (dissonances on strong beats are more emphatic, those on weake beats less vital), and to how they lie within the phrase (dissonances tend to resolve at phrase's end).