That’s very likely. The cables should be marked L1, L2, L3 and N, although as you have the old colours, the installation probably predates this practice. Done properly, the 3 reds should be red, yellow & blue.
No. It will give you the sum, but the vector sum. If the loads on the three phases are balanced, which they should be but never will be in practice, the neutral current should be zero.
The only way is to do it properly - measure each phase separately and add the three average powers. You’d be interested in the emonTx V4: AVR-DB: emonTx V4, new hardware in progress - #90 by Swwils