3000 watts at 12v is 250 amps. The inverter is 90% efficient so that bumps it up to 278 amps. Usually it's prudent to size fuses about 20% larger than your expected max load, or 20% smaller than the safe current for your wiring, whichever is lower. You haven't told us anything about your wiring, but based on the loads, I'd fuse at 325 amps. Sizing the shunt for the same would work fine.