When you say 3 RCA's im assuming you mean pairs (6ch). Yes they make adapters for all apps. I don't know why noone answered this question for 23 hours.. Maybe cuz the way you worded it. Your kinda saying it bass ackwards. your feeding ur amps from ur H.U., not the other way around.. whatever RCA pre-outs ur head unit has dictates whether u need any adapters..
some amps have an RCA output/thru so you can daisy-chain. some 4 channel amps have a 2 channel input mode so you don't need all four.
tell us amp make/model numbers and/or provide links or pics to the controls sections so we can help identify what you NEED.
you can get Y adapters for RCA cables. you would want a Male to 2 Female RCA adapter for both left and right. to split it to 6 channels, you would need three of those per side for a total of 6. the HU might not like it since you are paralleling the load seen by the HU.