Actually ur 2 key points
1) Forseeable Single forever
2) Budget is kinda stretch for 3 rooms...

Go for 2 rooms then and have a financial easy life.
Since is ur house and privacy, dont think u will wan to rent out ur room just for income to cover ur instalment (if 3R)
Guest is only once in a blue moon, dont put this as main factor (except if ur guest is ur parents which comes frequently, different story)
Its tiring to keep & clean a third room.