It depends on the location. Some locations require you to return every 28-30 days and they re-write the contract while others don't seem to care and you can just keep the car and then you are charged at the end for the entire period. If you don't (have to) return to the location during the rental then it's a single multi-month contract.

As far as credit card insurance goes, if your card specifies a maximum rental duration, and you exceed that duration, and then subsequently have an accident that leads to a claim being filed, there is a decent likelihood that they will deny the claim. It really depends on the adjuster and whether they notice or not.

Note that even if it was written as 3 separate contracts or you originally reserved 3 separate rentals, back-to-back rentals are considered a single rental, so there's still a chance a claim would be denied unless you switched credit cards each month. Again, YMMV but it's probably not something you'd want to take a chance with.