Hi alexjsc, One way to power extra fans is to tap off 12v from one of your 4 pin molex connections, to, say a DVD drive.
There are molex adapters which have a plug on one side and a socket on the other, plus a small plug for one of your extra fans.
You need to check the number of pins on your fans connections, could be 3 or 4, but you need power for 3 fans also.
Also, do you want to be able to slow the new case fans, or to have them running full speed always.
This is the type of connector;