What is the most important thing about being a parent of a three year old?
This is one of time's most complicated questions, honestly. I think the most important thing about being a parent to a three year old is loving them, but the tricky part is how you let that love manifest. Kids are super sensitive and still learning emotions at this age, so its really important to be slow, intentional and extra super patient with a three year old. 

An example I have of this from my little girl, Addie, who just turned three on Sunday, is communicating what is possible vs. what is not possible in a very clear way. Last night, I told my daughter that we had a doctor's appointment in the AM for her annual check-up, but she immediately wanted to go THEN, right away to the doctor. There began the two hour downhill spin into a full tantrum because she did not understand that she could not go to the doctor that night, because they were closed.

This might seem like a disparate example of how to show you love a child, but it took every ounce of my patience, calm and rationality to keep explaining to my screaming, flailing, alligator of a child that we could not see the doctor until tomorrow. (Also, what three year old is THAT excited to see a doctor?  I don't know, its insane to me!)

So the answer is, love, but in all ways that you can manifest that love, directly and indirectly.