No, the 3Com 100BASE-FX multimode and the Cisco 1000BASE-SX multimode GBIC cannot communicate directly with one another.
Fiber optic ports are not like 10/100 or 10/100/1000 ports that can upshift or downshift their speeds to match what's at the other end. On a fiber port, what it is, is what it is. The only thing that varies is maybe the duplex setting: 100BASE-FX can be either half or full duplex; Cisco's SX GBIC is full duplex only.
If you need the 100BASE-FX fiber uplink modules in the 3Com switches to connect to the Cisco 2950 switch, then what you'll want to look into is Fast Ethernet media converters. These would be located near to the 2950 switch (within 100m). They take the fiber feed from the 3Com switch, and convert it into a 100BASE-TX connection that you can connect to an RJ-45 port in the Cisco switch using a regular Cat5 cable.
Transition Networks (www.transition.com) makes a variety of these, find the one with connectors that match the ones you already have on your patch cords. (Probably SC connectors.) They make single standalone units, or modular ones; and the larger modular chassis can be rack-mounted. There are other manufacturers of media converters, but these are the ones I use.
Hope this helps.
