You can't simply take all the 3 seperate 12V lines and combine them, not many PSU's work like that.
You'll commonly see it on dual rail PSUs (the ability to combine rails - Tagans do for example), but triple rail is slightly different (manufacture dependant, of course).
Pulling 52A down those cables would soon cause them to sizzle - literally
The important thing that needs to be checked is the maximum amps that can be combined on a single line.
The oldies here will remember the first dual-rail PSUs and their inability to combine rails which caused systems to not even POST because of the high demands of the graphics card. While its unlikely to see this today, It just illustrates that you can't do rail1+rail2+rail3 = usable ampage on one rail
The maximum available on one rail should be in the specifications.