Errr... yes and no!
Every power supply is rated for the total power that it puts out. These are spread over the + 3.3V, +5V, -5V, +12V and -12V. Every PSU is different.
P=VI (Power=Voltage*amperage)
You have to check your PSUs for their rating. You shouldn't have a problem hooking up 3 PSU, to make 15V, but your amps are still limited at the same rating (35 amps?) of one of them.
Personally, I don't like using PSUs, because they are too bulky, and because they are made to provide 5 different voltages, where I only need one. Being a cheap bastard myself, I'll make my own.
...but it will work.