I was heavy into Flanker a couple of years ago so Im pretty sure about this.
Flanker 2.5 recognizes only one gameport configuration.
So if you have an old fashion gameport CH setup it should work all linked together, & plugged into the one computer gameport.
However since you say it does not, Im assuming you have the newer USB setup which will not work with more than the one Flightstick since the three devices each use their own gameport connection for a total of three which is two more than Flanker recognizes.
I know of no way to hook up all three USB systems to Flanker.
One thing you might want to google is an adapter. They do make gameport to USB adapters in which you can hook up a gameport type setup to an USB computer so maybe they make a gaming adapter where you can hookup 3 USB devices to one computer USB connection.
Another idea is to buy Lock-On, Modern Air Combat which includes the same company's Su-27, & Mig 29, as well as the F15c which does support modern multiple USB connections.
No flyable F4 Phantom though - though they're out there for your Russian plane to shoot down.