Running 3 fans off one molex is fine (Especially if they run at 7V) . You could safely run 5-6 fans before your current starts dropping.
actually, you can get a splitter and run all your hd's and cd roms and fans off one psu header since if you've ever opened one up you would see that they all trace back to one solder joint... or rather, a cluster of them.
the power drain depends on your psu, so if you have a 500watt vantec or something you can run a TON of fans. i am not correcting you, just giving some info so dont take it the wrong way. sometimes these boards is the only way you learn. go get a psu from an old 486 and take it apart. you can learn alot from them