On New World Macs, the ones which have USB ports built-in, the boot ROM is on flash chips and is totally updatable.
I know there are several official firmware updates, but I forget what computers they are for.
If your G3 doesn't have built-in USB, then it is an Old World machine and the ROM chips can't be reflashed. In that case, you would have to replace them.
When Apple does have to fix bugs in Old World machines, they have the System Software put code in the NVRAM, where it is run very very early in the boot process.
I believe that the actual benefits to updating the ROM depend on the particular bugs in that ROM version. They probably aren't that important, as your machine can boot at the moment.
In theory, one could write a firmware update or NVRAM patch that does almost anything. In practice, we don't know enough about Open Firmware and the super-early boot process to do that.