Since the Mz3 is made for regular and doesn't have an electronic knock sensor, it will not gain any performance nor MPG from premium. On the other hand, if you put premium in a car with an electronic knock sensor, you might gain a little performance or fuel economy (try it on a Honda Accord V6 or 4 cylinder as an example).
Also, running 91 in your 3 will NOT make it self destruct next week. It might, over time and many tens of thousands of miles, build up more carbon than it would on a strict diet of 87. The degree of carbon buildup all depends on things like combustion chamber design, piston design, valve design, combustion characteristics, sparkplug placement, ignition performance, etc. etc. etc. Some engines will show a good chunk more buildup than others, and some will be more adversely affected than others.
Engines that routinely run for short periods and at low engine speeds in theory should build up carbon more quickly. If this is you, start running on 87.
My college shop teacher worked on E-Type jags back in the 70's. He told stories about customers would bring their cars in because they ran badly. He and the other mechanics simply took them out on the freeway and did a few high RPM blasts up the local grade in 3rd gear. They blew out the carbon deposits (no cats, don't forget), put new plugs in them, and they ran like champs.