I realize that this topic has already gotten to page 11, but I wanted to say Thank you, Goofyluvr, for posting that blog on page 1. It was enjoyable to read.
My son is almost 5 and has a 7 year old sister, so he has been exposed to a lot of princess and other "girlie" stuff. He loves the princess, Barbie and Bratz Kids DVDs, and he enjoys playing princess board games with his sister. He does, however, hate the dolls because their doll faces and legs freak him out.
Recently at Build a Bear he bought a rainbow covered bear because he thought it was pretty, even though the bear was clearly marketed towards girls.
Whenever I paint DD's nails and toenails, he insists on getting his toes done, too. So, today, he got purple dots and he proudly showed them off at the pool. He did get teased by some other boys, so when we got home he wanted them removed. He's very sensitive to what others think of him, which does make me a little sad, because I don't want him to stifle who he is or not follow his instincts because he's worried about what others might say.
Despite all this, he will not be caught dead wearing anything princess or any other "girl stuff." When he was 3, however, he pranced around for weeks in DD's Tinker Bell costume that she had outgrown, whacking his sister with his Nerf sword and fighting the bad guys. He needed to wear the Tinker Bell costume because he needed to fly away from the bad guys faster. We used to get all sorts of comments from my in-laws about it, but I thought that if he was happy and didn't see anything wrong with it, then so what. Let him wear the stuff and be happy. I knew he'd probably outgrow it sometime, which he is starting to. I didn't let him wear the Tinker Bell costume out of the house, but yet I didn't let DD wear her dress-up costumes out of the house, either. My in-laws make comments all the time about DS growing up to be gay and how I'm teaching him that it's OK to be "femmy." I could really care less because DS enjoyed himself and wasn't hurting anyone, so what's the harm?
I'd say to the OP, let your son wear whatever he wants to wear, whatever will make him comfortable. If anyone makes comments to you about your 3 year old boy dressed like a girl, you should feel sorry for them and for their narrow-minded hang-ups. Who cares? He's not hurting anyone, so what's the big deal how he's dressed! But just make sure you get lots of pictures!