If you are doing the work here in the US ... pls provide the following additional details ...
1) What size service do you have to your main pnl and where exactly is the main pnl located?
2) Where exactly is the FPE sub-pnl located?
3) What sort of loads are you pulling on the FPE sub-pnl?
4) What is the distance between your main pnl and the FPE sub-pnl?
5) Are the 3 existing sub-pnl feeded conductors in conduit?
6) What size conduit?
7) What size are existing 3 sub-pnl conductors?
8) What size sub-pnl are you planning to install?
9) Have pulled the permits for this work yet and have you verified with the AHJ the need for Arc Flash Breakers on your branch circuits in the new sub-pnl?
Depending on your answers ... I might have a few more questions or comments.