Any reason why they aren't all on the same subnet (i.e. 192.168.0.###)?
If you do need to keep them on separate subnets for some reason, you might have to add a static route between the two subnets on Red B to point to the server on Red A. Your subnet masks instruct each router to look only within its own subnet, and then it goes to the gateway which routes it to the WAN instead of looking into the other subnet.
Correct me if I'm wrong, but the WRT54GL is based on BROADCOM BCM5352, and this is the BROADCOM Forum?
Of course, we have no idea what the "other 2 networks" are in terms of hardware, a pair of WRT54GLs, switches connecting PCs, a bunch of cans attached to strings with kittens playing with the strings... :shrug: