Just in case you are still interested in a driver for your 3Com adapter.
I had compiled and posted a 3c59x driver for ESXi 5 at viewtopic.php?f=24&t=3527
months ago. This is the driver I personally use on my own ESXi 5 installation. My network adpater is a 3Com 3c905TX (Boomerang). So far the driver performs well without problems.