3.9 will work on that setup, but you might find your RAM doesn't stretch as far as it does with 3.1 (3.9 needs more RAM). You can see how much RAM is on your accelerator using ShowConfig or SysInfo - it will show all the different contiguous RAM blocks. The Supra RAM will show as 16bit, the GVP RAM will show as 32bit.
But by no means is 3.9 required for TCP/IP. Miami, AmiTCP, and Roadshow will all work under 3.1.