Generally mixing puffers are not recommended. Just cross your fingers and hope it will work out in the long-term. You know what to do if one of them gets aggressive.. I know some puffer experts who mix puffers together while keeping careful watch every single day for aggressiveness.
A 180g system can handle all 3 puffers. If you add the 3rd puffer, then I don't think you should add a 4th puffer. You might be able to add another Canthigaster species puffer, but then you'll be really pushing it. Most dogface puffers are mellow, but canthigaster species are less forgiving toward their own class of puffer.
With a 180g, I would do a porcupine and dogface puffer tank. Both will end up occupying the WHOLE tank as adults and very personable.
In a FO, you really need to keep up with those expensive water changes. Why don't you look into getting ~250lbs of live rocks, a skimmer (if you don't already have one), and a refugium. This really lesson the expense of doing those weekly 50% pwc in FO systems. Upfront cost is a lot, but you'll save more money in the long-term.
