Bogdan,
what you are describing is more more multiple sip accounts per one auth account than aliases (aliases are only for inbound traffic).
if you want to have multiple SIP accounts (A, B, C) to use the same credentials, take a look at uri_db module: http://www.openser.org/docs/modules/1.2.x/uri_db.html
of course, you still need the alias support for the inbound part.
Ah, of course! I forgot about uri_db, thanks for pointing it out to me.