and white's right you might be loosing a bit of power going to the one amp, but i dont think it'll be that drastic of a difference. those rf's are underated but im not sure about by 400w, who knows i may be wrong. I know the nice old school 1000a2 's were underrated by about that much, and the T series big rf's are also grossly underated...not sure about the newer P1000's though, i doubt they're as underated, but without test results in front of me i can't say for sure.
(1) 3k amp should be more efficient than (3) 1kw amps...how's your electrical looking?