Three Year Old's

What do you do with a three year old. Mine's name is Ava and she is three and a half. The me do it myself stage has lasted a long time, I am ready for it to be over. Usually, depending on the day, we have some sort of episode. Today it was this: I asked Caroline to carry to jugs of milk into the house from the car. I had to carry Margaret and Ava, because she never wears shoes. I didn't want to go back to the car just to bring in the milk, so I thought Caroline could do it. Ava decided that she wanted the challenge. So after crying for a long time, I of course put her boots on, took her back out to the car and put the milk back in the car so she could carry both of them into the house. Of course both were to heavy so I carried one and she managed to bring the other into the house. Happy as a little plum now. So this is what I always do, I don't want to do it at firts then I always give retrace what the problem is and let her redo what she wants to do. It usually makes her happy, but me mad, because I have to give in to her. So what should I do? Give in or listen to her cry for a long time, and she will cry for a very long time. I guess it is because I just want to get it done and don't want to take the time for her to do it. I will never forget the middle of the night when she got up to go to the bathroom and I, not thinking, flushed the toilet. This act is not allowed in Ava's world, she always has to flush. So there we sat for a good while, her bawling, until I put her back on the toilet, pretended to wipe her and then she flushed. Happily she went back to bed. Three year old's are exasperating.