Yes ofcourse breaker is 3 phase and also 1250AF (at full load) with L, S, I and G functions made by ABB.
We are using here IEC codes.
In my opinion 300mm2 cable is enough but how many parallel conductors and how many cores?
Oh, you are using IEC code and do you have any cable catalog or have you requested any company to send you their cable catalog.
what type of IEC cable you are using have you noticed this. In NEC there is ThHHN, THWN and many others.
IEC also has NYY and many other types which current capacity differs per type.
You should have IEC cable catalog from a company.
you can not gues like this 300m sq is enough and how many.
There are many other factors to be considered like voltage drop, derating and etc.
Try to you use NEC as a guide book for factors.
I will give one IEC cable catalog please do not just take cable ampere capacity also consider voltage drop, derating factor, and others.
If you still have any farther question please let us know.
By the way, you are working in Sudia and you are from Pakistan. Which part of pakistan you are