Read the Building Blocks page about using the emonTx in North America. There you will find lists of suitable current transformers and details of the changes that you will need to make to the emonTx.
(I am assuming here that you actually need to measure beyond the 100 A that our ‘standard’ CT caters for.)
Maybe, or maybe not. Obviously, @Abhishek_Singh is there and can measure them, and the aperture sizes are quoted, I think. I understood he’s not in the USA, as his supply is 440 V, 3-phase. According to me, 35 mm2 PVC insulated/PVC sheathed should go through the YHDC CT. (Its o.d. is 12 mm, the CT aperture is 13 x 13 mm.)
As far as I know, YHDC do not do a 125 A CT, however, the SCT-013-000 shows only slight non-linearity at 120% of rated current, so you might like to consider that and decide whether you actally need to measure up to the full 125 A - or indeed whether you need to measure above that, as most supplies will accept a moderate overload in the short term.
In order to use that ‘extra’ range, you will need to lower the value of the burden resistor. How to calculate that is in Building Blocks, without knowing what the rest of your circuit is, I cannot help any further.
That depends on whether you want to measure Apparent Power (VA) or Real Power (W). If you are content with apparent power, then you multiply the rms line current by the rms line-neutral voltage for each phase, and add those numbers together for the total. If you need real power, you should sample voltage and current simultaneously, multiply those to give you instantaneous power, then average the power over a period of time, and finally add those to get the average total power.
Again, Building Blocks gives you the maths, and the sketches if you want to use an emonTx or Arduino, for those calculations.