MÆGGES,
I have some ideas.
1. [REMOVED, IRRELEVANT, re pixel line-up]
2. Your stock PLP resolution (ie if not dropped res on sides) is high for R9 380X. Card may be struggling to keep up (but I doubt this is the source of your tearing).
3. Tests:A. Try spreading a game manually, using SoftTH fullscreen & Eyefinity Off. Still tearing?
i. This would prove whether or not Eyefinity is at fault for tearing.
ii. To find a suitable game & spread instructions, visit PLP Gaming wiki & go to Manual PLP Games List. You want a game that says "Fullscreen" in pink, at its top.
B. Back to Eyefinity (not SoftTH). Try VGA on the sides, if you can. See if it's better, VGA is pretty forgiving connection.
C. Drop ALL monitors' resolution below stock to get your gaming pixel-count low. Does it still tear?
4. What monitor connections do you use, & what adapters? How long are your cords?
5. Consider FreeSync, would likely fix (e.g monitors supported:
http://support.amd.com/en-us/search/faq/284 )
I don't have Eyefinity PLP yet, have not tested it. But I know other people are happy, so you can likely fix tearing somehow... I'm pretty sure it's not the R9 380X itself, but something in-between the 380X & your outputted game. Sounds like connecters & cords.
As for disappearing mouse, this does seem to be an ongoing issue in some games. Some people complain about it, other people don't seem to have the issue (or are playing different games).
Once you get your PLP setup working without tearing. For games that have other issues (e.g. no mouse, breaking game), consider using Manual PLP methods.