we know the original 828 + motu 828 mk2 + are mac os 9 compatible..
but the real question is..

the mk3 - is this also compatible with os 9?
this is a question id like to know the answer to!

and the answer is:
http://www.motu.com/techsupport/technotes/828mk3-hybrid-system-requirements
doesnt look like it;( unfortunately this page claims that it requires 10.5.8 leopard at a minimum!!!!!