multi gpu boot issue

i have an evga x58 classified motherboard with 3 GTX 970s in it. it seems fine to boot any two cards in any two slots, but if i add the third card boot halts at "Booting the kernel". my kernel is configured to max out at 16 gpus, and im not sure what other provision needs to be made. x does not automatically start with my system, i always manually invoke it with 'startx', so my issue ought not be x related. does anyone else have a multi gpu rig on crux?