Having repaired two REL subs myself, a Stadium lll and a Strata lll, I'm fairly certain the B3 has a very similar amplifier PCB judging from the B3 description in the operators manual. There are three(3) fuses that need to be checked on your B3. The first fuse is the one on the rear panel in the IEC connector, the other two are only accessible by removing the rear panel operator interface/amp assembly from the enclosure. These two will be located on the amplifier PCB and are for each PS rail right after the mains transformer.
Check these fuses and while you have the interface/amp assembly removed, checked for any burnt or discolored components on the amplifier PCB as this is where most failures occur. If you find no obvious component failure(s)or suspected failure(s) but you find an open fuse, replace the fuse with the same value/rating then replace the interface/amp assembly and reapply power. If she lights up, you're good to go.
Let us know what you find and we can go from there if you want. BTW, REL holds their designs and component values as proprietary and will not give out any info nor will the dealers. They will insist you send the unit in to them for repairs/troubleshooting.