PSU 12v rail graphics card power distribution?

I have a GTX 480 and the manufacturer recommends 42 amps on the 12v rail. I have three that are rated at 14, 23, and 15 amps, which is 52 amps. Should I spread the power out over the three rails to get as much amperage as I can as opposed to having it on one rail? Can anyone think of anything bad happening? I know the purpose of having more than one rail is to have everything separate but I just wondered if this is ok. I don't know much about power supplies.