Of coarse youll be splitting the signal so each amp will be getting the equivelant of your guitar with the volume turned down significantly. Its something that is done for two amps but should never be done for more then 2.
At that point youd have to find a good buffer/splitter box that converts the guitar signal to a Low Impedance signal that can feed several amps without significant degredation and excesive noise. Theirs a few companies making these things, the most famouse is the Framptone Amp Switcher. That only works for 2 amps though. http://www.framptone.com/
People use two of those units to go to 3 amps though.
The market for multiple output splitter boxes is very small and specialized. Ive seen them but couldnt say who makes some off the top of my head.
Its also VERY common to run into grounding problems when your plugging into more then 1 amp. Its VERY common and requires a phase reversal switch or jumper on the splitter. Otherwise youll get a very loud buzzing on one or both amps.
Its something I do alot when Im recording. My favorite high gain sound right now is a Bogner XTC + Mesa Triaxis. Ive also done Marshall Plexi + Bogner, Plexi + Triaxis, Plexi + AC30, and mixed more then 2 fenders. Youll really like the effect. Try panning one hard left and the other hard right for a real cool effect.
I hope that helped. With 2 amps Id still say use a buffered spliter but a stereo Y cable can still work. Anything more and youll have to look into buffered spliters.