What you are supposed to do, is to tell all the firms you have dealing with that you are resident in Canada for the next three years. However, three years doesn't strike me as very long and that is going to be a bureaucratic hassle (some companies might not even let you keep your accounts open).
The important thing is that if someone writes to your address, that the mail gets to you reasonable promptly.
If you know your tenants personally, you could ask them to forward post to you (or possibly to open it and scan it to you). I wouldn't do that if you don't know them - important stuff may build up over a long period.
Otherwise, do you know anyone in the UK that would be prepared to do that for you (parents, siblings, close friends)? Changing your address to their (UK) address is probably straightforward and unlikely to cause problems.
In terms of credit on your return, I wouldn't worry too much. I presume you have a mortgage on the property which you are keeping up with; that should be enough to maintain your credit score.
P.S. If you have a driving license, don't forget to tell DVLC.