Hello there. I don't think pressure altimeters have comparators in EICAS aeroplanes at all. They can differ quite markedly if they vary either side of the correct figure- if you can work out what the correct figure is! There is a lot of error in pressure altimeters judging by the variations you get across the three, so comparators would end up working rather hard!
Procedure is to do an RA check at 2500'RA when they come alive. We do RVSM checks every hour every flight on the PAs. Everywhere seems to have gone RVSM, even Africa (God forbid!)