Your fathers condition (speech wise) is known as Expressive Aphasia and results in a severe impairment of speech and language abilities. Word finding difficulty is one of the chief difficulties but basic language concepts are also affected so a person might not understand numbers or recognize written words. Although a minimal ability in the areas can often be seen. Automatic speech (like during counting aloud, or singing a song or reciting a verse) is but to no real practical advantage.
Since your fathers condition is 3 years old, the period of major spontaneous recovery is passed and although speech therapy might result in some improvement, the progress will be slow and slippery. Therapy at this stage is helpful to occupy the person and to minimise emotional lability which is often seen in most cases. Therapy will keep up morale and chart progress achieved. It can also enable him to communicate needs of daily living through development of communication system which uses the abilities he currently has.