I was randomly browsing questions on this site, and saw one about if a kidney was removed, answered by Mike(?) saying he only has one, and that it is larger and picked up the slack of the others.
When I was younger and had x-rays done, I found out I have 3 kidneys (a trait my father shares). I was wondering if this is somewhat common, and what the effects would be?

It depends what was meant by three kidneys. there are very many possibilities and some affect kidney function though most don't and cause no problems at all.

This is not a medical advice site so my suggestion would be you see your GP/primary care physician and request a simple blood test to look at kidney function ad have an ultrasound of the kidneys just to check what is going on.