I have been researching the 3 brands extensively, as I just purchased a full size, and my dad is looking to get a commander-size.
Armscor makes RIA and Citidel, Metro Arms makes American Classic, and Shooter's Arms Manufacturing (SAM) make the ATI pistols. They are completely different companies.
To generalize: All three companies are dependent upon their American distributors (I believe all three are operated independently from their respective Philippine manufacturers) for customer service in the US.
The Armscorp line has been sold in the US for a longer period of time. Their RIA line has an excellent reputation for customer service in the U.S. The pistols are perfectly functional and utilitarian, but not quite as pretty as higher-end ones. I just bought one, and can attest to the fact that they have a few rough/sharp edges that I would not have liked to be there, but are otherwise tight, accurate, and function perfectly.
Both ATI and AC are newer to the US market. AC had some customer service growing pains over the last year or so (horror stories, to be exact), but from people I trust, I hear they have worked things out. I have only heard positives about ATI--in fact they just hired a top executive from STI (which is one of the most respected higher-end production 1911s).
To offset their shorter track records, AC and ATI are a bit less expensive than comparable RIAs. Along with lower prices, you also tend to get a bit more features. The biggest one is that with AC, you get a higher-end looking gun, with better metal finishing and a shiny deep blued finish instead of the dull black parkerizing of RIA and ATI. AC also has fully-forged frame and slide--which may not affect daily operation, but is considered highly desirable.
Each company's models have a slightly different set of features on each equivalent product. None are necessarily wrong, but they are different. E.g., the RIA mid-sized pistol is not really a true Commander, since it has a 4" bull barrel (in contrast to AC and ATI's 4.25" bushing barrel Commander models. Folks have preferences for both variations.
I don't think you can go wrong with any of the choices, but it is a matter of choosing the one that matches your priorities. I've had "better" pistols, but I have never had more fun than I am having with my RIA Tactical. Good luck!