Your dentist doesn't need to be a "special" pediatric dentist. Ours isn't. And our kids have never had a problem going (starting also around age 3) and our youngest actually looked forward to getting to go. They are 15 and 12 and still don't mind. Our dentist has a good chair-side manner and is professional, competent, and funny! And he's a dad. So he gets kids...
I never made it into a big deal for them. Just something we do twice a year to make sure we are taking good care of our teeth. Like you go change the oil in the car, or whatever. No drama required.
We are a pretty low-key, non-drama kind of family. So that was pretty easy.
I also read them The Berenstein Bears book about going to the dentist. Not as a "prep" book, but just one of many BB books (and other books) that we read as bedtime stories. I'd pick a new book every few weeks at the book store (or several) and they would help choose them also... and then at bedtime, they'd get to choose what book to have read to them. Sometimes I chose one and they chose one, etc.
They liked the story. How much it affected their early dental experiences, I can't really say... but we never had any issues. Still don't, and I have one almost out of braces, and one that has had to have several teeth pulled (for orthodontal reasons), and they still have no problems with the dentist.
http://www.amazon.com/The-Berenstain-Bears-Visit-Dentist/...