Hi Ian !
Guest rating is exact;ly what it sounds like, it gives you the ability to rate your guests on a 1 to 3 scale ! It's up to you if 1 is good or 3 as in three star guest.
You can choose to email all your guests who stayed in March who were rated 3 and send them a nice thank you for staying email.
This could also feature a link to your tripadvisor page exhorting them to give YOU a good rating!
As they are rated 3 then this must have been because you liked them and you felt they had a positive experience, basically you're accentuating the positive to your already happy guests!
This way it will help you attain more positive feedback on your trip advisor which in turn will turn into tangible benefits later.

Cheers
Patrick