People like to scare the parents of two-year-olds by telling them that three-year-olds are worse.
However, they feel right. Because a two-year-old has the advantage of still looking like a baby. An amazingly big and mature and capable baby. One that can walk--at least a little ways--and communicate--however basically. When they still act like a baby, you are not surprised. When they act a little more grown-up, you are thrilled.
A three-year-old has longer arms and legs and dirtier feet and firmer lines. A three-year-old looks like a child. A very small and helpless child. And instead of being amazed at how far you have come, you start wondering why you aren't any farther.
You stop being thrilled they went on the potty and start wondering why they can't make it through a church service without three trips (in the case of twins, each). You ditch your strollers and diaper bags and then wish you hadn't. You think you can skip naptime and find out you can't. You forget that you can't actually understand that three-paragraph discourse unless you stop to listen and then you get a why-don't-you-understand-me tantrum.
Three is not so bad, if you don't buy too heavily into your own propaganda that they are big kids now.