The IRQ used by the 3C503 or 3C503-16 board depends on the I/O address set using the jumpers. For example, if you configure the 3C503 or 3C503-16 at I/O address 0x250, no other devices in your system should use IRQ 3.
The 3C503 board has a limited amount of on-board memory, which causes very poor NFS software performance. To avoid this problem, use a 4-Kbyte read and write buffer size to mount the NFS software over the 3C503 interface (see the mount_nfs(1M) man page).