What are the chances of getting skin cancer from tanning bed if you only do it 15-20 minutes 3 times a week?

Depends on duration. The main question is for how many years? Clearly cumulative exposure to tanning beds at a young age is a major risk for melanoma, especially for fair-skinned and red haired individuals. Melanoma cases have been rising for years, for a variety of reasons. My advice is to not use tanning beds at all-- it's not worth the risk!
Higher than average. Would recommend you stop. We think it is cumulative exposure, so a little still exposes you to unnecessary risk.