Three Visa Exempt entries over 6-7 months might not be be seen as tourist behaviour by your Immigration Officer so be prepared to be either grilled or even denied. You might want to consider the Multi-entry Tourist Visa especially if the Visa Exempt does revert to 30 days any time in the near future.
It is the landlord's notification to Immigration of a foreigner staying. The proof of address is the residence certificate obtained from Immigration which most banks require.
90 days is easy as there a few ways to get a 90 day stay without leaving Thailand (Tourist Visa (and, currently, Visa Exempt), 60 days + 30 days extension, Non-O over 50 (90 days)). Will you be visiting once a year or more because the Non-O over 50 plus the 12 month extension might suit such a plan?