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The +12V and -12V rails are separate power busses, they dont' add together - you should be okay with those numbers as stated, though the common advice is to leave some headroom between your usage numbers and the PSU's capacity numbers, so you don't want to push it all the way to 3A on either.

You can't directly daisy-chain modules, but you can either make custom multi-drop power cables yourself or buy some "flying bus" cables which plug into one socket and have more sockets on them for individual cables. In my rack I made a few custom multi-cables just to simplify the cable mess inside! You don't want to chain power-heavy modules this way though, only the "small" ones, my biggest grouped cable is 4 modules totaling ~600ma on the +12v.