This is a pretty classic prisoner's dilemma. Anyone who defects earns $3k more a month and has no appreciable affect on the rent for any other units they may ow

This is a pretty classic prisoner's dilemma. Anyone who defects earns $3k more a month and has no appreciable affect on the rent for any other units they may own. Unless landlords are very unusually scrupulous, the resulting equilibrium won't look like a monopoly.