There is no such thing as Fancy Pants 3.So - There is no website that you can play it on.
Fancy Pants Adventure 3 is called Sneak Peak. And this is the website where you can play it. http://www.playhub.com/adventure-games/6632/Fancy-Pants-Adventure---Sneak-Peek.html
One can play the Fancy Pants game via the Armor Games website where one can play the game online. Alternatively one can play on the Play Fancy Pants Online site.
I edited the last persons answer because they were wrong. Fancy pants 3, by Brad Borne is currently being made. you can keep up on his site, bornegames.com
There is no boss in fancy pants 3.
fancy pants 3 is on armor games and kongregate
February 2011
There is no rainbow colored pants on Fancy Pants Adventure 2. But maybe there will be in adventure 3!
addictinggames.com
http://www.playhub.com/adventure-games/6632/Fancy-Pants-Adventure---Sneak-Peek.html
there is only 2, no 3
you can find it on armorgames.com