Great tenants, but want to sell house

[Marci Stein]

I’ve had beautiful tenants for 8 years - 

never once late with paying rent, 

but I am ready to sell the house.
The lease ends in 3 months.

What’s the best way to let them know I’d like them to move because I’m selling ?
They cannot afford to buy it. 
should i go month to month or not renew at all ? Thanks very much!