Bingo !

Found why my 3c900A could not work.
Tried to set the irq manually.wrong.
and Bus mastering was disabled what
I change for my netcard slot 3 to enable.

and more info.:
from 3Com
"Should the IRQ Be Changed?

NOTE: 3Com recommends assigning its NICs to IRQs 10, 11 or 12
because these are often unused. IRQs 2, 9, and 15 are not recommended
because they are often used by other PCI devices.

NOTE: NICs must be installed in slots that support busmastering, and busmastering must be enabled
within the BIOS. "