When you have two different make PBX's, it is not so much "integrating" them, it is simply getting them to communicate with each other (a call passing from one to the other). Much the same, as if they were located on different cities.
The three basic "ways" are (and how each is done, depends on the technology used on the Non-3CX PBX)...
Extension from one connected to a trunk of the other.
Trunk from one connected to an extension of the other.
Making use of the Bridge Trunks (in 3CX), for trunk to trunk communication.
A lot of this assumes that the other PBX is a SIP based piece of equipment, and not proprietary, or you will require ATA's and gateways (if the other end is analogue).
