Disconnect the hard drive and power up the system. Make sure you plug in the zorro daughterboard as well, because A3000 don't boot up without it plugged in.
If you get the "Insert Disk" screen, then probably 3.1 roms do not like previously installed 2.04 session on the hard drive.
If you don't want to test it like suggested, then simply power up your machine as is, and hold both mouse buttons immediately after power up and see if you will get into the early boot menu of the kickstart roms.
