SAML for o365, google apps and cloud proxy

Yes, you most certainly can have it all done on a single IDP. What you do is just take your IDP configurations you already have defined and just consolidated them to a single policy - i.e. assign all SAML resoures to a single APM policy. Of course, you will probably need to update your SAML configs/meta on SP to account for the same ACS instead of using three different ACS URLs - but that should be it. You can run through the iApp as a dummy and see what kind of config it builds when configuring multiple SPs to be federated by a single IDP if you want to be sure your config is exact match of what the iApp creates.