The best thing I did when DS was three? I read
Playful Parenting by Lawrence Cohen. Joining your DD in her world as often as you can, will help her join you in yours when you need her to. And when I say, "join her in her world" I mean CHILD DIRECTED play. When I play with DS, he gets to be in charge. If the household is my domain, then play is his. When I sit down with him, I let him call the shots, boss me around, and we get silly silly silly. Someone posted on a thread least week about a physchologist who did a study and found that 20 minutes, 3 times per week of child directed play with the parent, greatly reduces disciplnary issues. I wholeheartedly agree and would add that it helps DS open his eyes and ears to me which is helpful in this "I'm focused and can't hear you at all" age. In actuality, I try for at least 20/30 minutes A DAY if not more.

Aside from that, relax and have fun. I found it was way too easy to raise my expectations at this age because DS became so verbally articulate. I had to remind myself that he was still such a little guy, emotionally driven and raw. To change the way I saw things, rather than trying to change behavior was a huge help.
The best part of this age is the pretend/fantasy play. DS became the ultimate ROLE PLAY kid at this age (still there in full force at 4 1/2). He spent much of the year as a baby, actually. And there is one more thing thing that was helpful for me to keep in mind: 3 is that crazy age between baby and child, it can be a bit of a muddle for kids. They want to be older, but lament the passing of babyhood. When DS needed to be babied, I gave him what he needed, and when I sensed he needed to the big guy, I found new ways to empower him.
In general, 3 is just more of the same: following cues, coming close when needed, and backing off when needed. And play. LOTS of amazing play.
Have fun!
Em
P.S. I think the age thing is so subjective and differs for all kids. DS at 2 and 3, was a rather predictable thing. THere were colorful behaviors for sure, but they tended to be fleeting and so normal and predictable. Now four? That's another story entirely...
