What kind of GBIC or GBICs installed in the 3508G?
What kind of 3Com 3300 switch is it? (3C169_____)
On the 3Com switch, are you trying to connect to a built-in fiber port on the front panel? Or an expansion module port on the back panel? And if it's an expansion module port, what's the 3C-number of the module? (3C169____)
Without those details, I can only tell you what your fiber options are on the 3Com side, and what they can connect to.
Physically it should work if you match speed and duplex (Gigabit Ethernet, full duplex), wavelength used on the fiber transmission (850nm for SX, 1300nm for LX), and correct fiber type and distance between the two (SX-to-SX uses 62.5/125 multimode fiber to 220-275m distance, or 50/125 multimode fiber to 500-550m; LX-to-LX can use multimode to 550m, or single mode fiber to 5km standard, 10km typical real-world).
And you will probably have to make the Cisco switch "less intelligent" so that it will work with the 3Com. (The Cisco switch will want to negotiate some things, but the 3Com switch won't respond to that, so you have to just tell the Cisco switch what to do instead of letting it learn for itself.)
The 3300 SM (3C16987A) has a 1000BASE-SX port on the front panel, with an MT-RJ connector. This will work with a 1000BASE-SX GBIC (WS-G5484) in the 3508G.
The 1000BASE-SX module (3C16975) that goes in the back of the full-size 3300 switches (3C16980A 24-port or 3C16981A 12-port) has SC connectors. It will also work with the 1000BASE-SX GBIC in the 3508G.
The 1000BASE-LX module (3C16973) has SC connectors too, but must be patched into a 1000BASE-LX/LH GBIC (WS-G5486) on the 3508G to work. It will not work with the 1000BASE-SX GBIC.
The 3300 FX switch (3C16982) has 8 ports of 100BASE-FX with SC connectors on the front panel. It will NOT work with any GBICs on the 3508G, because 100BASE-FX is Fast Ethernet only. It is NOT Gigabit Ethernet. The only way you can get a Gigabit fiber connection to a 3000 FX is if you have one of the Gigabit modules (SX or LX) in the back.
