I would reduce the time in the city to 2 hours as there is immigration, on a working day traffic is often heavy in the moning and you should be back at the airport at least 1 hour before your flight leaves, I would even need more time as Iīm not the high-risk type.

In this time I would do what joppiesaus suggested and simply enjoy the flair of Munich. Consider that many monuments etc. donīt open before 10 am. If you are interested in art Asamkirche is not to be missed.