When mounting more than one Atmel onto a board, I take it that the power supply can be shared amongst say 3 chips, howver, what about the crystal?
Of course you can share the power supply, provided it can provide enough current to handle everything you are running off of it. If you are running multiple processors and other things, you have to add up the currents you are expecting to be able to size the power supply properly - don't just throw a regulator on there and hope.
As far as the crystal is concerned...
Let me say this - I know of at least one person who has done this successfully with -2- ATMegas; he had them running in tight sync with each other. Whether it could be expanded to more, it is unknown.
With that said, you -should not- do this; it isn't the proper way to provide an external synced clock to multiple devices. There are clock driver chips out there that -can- provide a synced external clock to the ATMega, but you would have to use avrdude to set the fuses properly to handle this application.