It's true that if you want OBIEE 12c with OBIA (any OBIA version) you need a separate install of OBIEE (so it isn't about upgrading the one out of the box).

That's what your machine 2) is for.

But doesn't mean you need to split Informatica & DAC away from OBIEE 11g. And because that OBIEE 11g isn't used I wouldn't plan way more RAM than that. It just needs the RAM to start and be up, nothing else as it will make no work. So let's count 1-2Gb for OBIEE 11g, must not be a big issue to have that spare RAM on the machine 3).

Good point: no idea if it isn't mentioned in the OBIEE 11.1.1.9 certification matrix because of "timing" (both 11.1.1.9 and SQL Server 2014 have been released almost at the same time, therefore no time to test and certify it).

This is maybe something where people having OBIA itself could have a better answer

PS: I asked for this thread to be moved to as it's an OBIA topic and would require inputs from these users.