It should be easy as you only need to define those networks with several crypto access-list on PIXA, eg:
acl 101 permit ip 192.168.0.0/24 10.0.0.0/24
acl 101 permit ip 192.168.1.0/24 10.0.0.0/24
acl 101 permit ip 192.168.2.0/24 10.0.0.0/24
Make sure you don`t have any routing problem with those networks insid the PIX A.
HTH