It maybe true that the app has improved and that one should simply disregard the reviewers on Amazon
that didn't like it. Personally, I stay away from 3 star electronics purchases, but you are exactly right, it is unfair to judge a product I don't own. If you buy something half the people didn't like, odds are you are asking for trouble...
It could be a great thermostat with hack-able options (probably not), but... I tend to only buy home automation products I know for sure I can interface later with other things. This usually means (in order of preference): a published ip based protocol, RS232, or IR. Even better are devices that do not require polling for a feedback response. For $360 (3 of the 3M50's), I would seriously consider a more open platform if that type of info is not available for the 3M50, but that is my opinion.
It's a mute point anyways, as the OP hasn't even said if there is a common wire at the three thermostat locations. I'm not going to read the 3M50 manual for the OP, but a thermostat is usually not a drop in easy install unless you know how to read a wiring diagram.
You simply can't always go off wire color. You must know what type of system you have now and how it's currently wired (stuff like is there a common wire, do I have single or two stage heating/cooling, etc...). It short, you really should know what you are doing and how to use a volt meter.