I don't think anyone is going to give you an absolute answer on 900MHz propogation, and certainly not Cambium as it would present a liability to them if they said yes it would work and then you go to buy and it didn't work. When dealing with near/non-LOS links there are so many variables that it's probably just best to buy some equipment to test with and see what performance you get. First step would be to get some noise floor readings from both the tower and at a prospective client site and see what the RF landscape looks like. Assuming that you have a pretty quiet noise floor, I think there's a decent chance of getting some of those clients within 3 miles some service... I doubt you'll be able to get anything to the people 5 miles away. Again, no way to know unless you buy some gear and test it out.
That being said, we've had some pretty good luck with the new PMP450i 900MHz gear.