I had made some mashed potatoes and I accidently left them out for 3 hours, were they safe to serve after being left out?

If they were just mashed potatoes, they were probably ok to leave out. However, the issue is usually what was mixed into the mashed potatoes milk & butter can be a problem with dairy more so than olive oil or margarine.

Here are the general guidelines:

Once cooked and left out at room temperature holding hot food needs to be 135°F or above and holding cold food should be at 41°F or below.

Is A Boiled Potato Left Out All Night Safe For Consumption?

Also from a web site on food safety called stilltasty.com “Big offenders include rice salads, pasta salads and that bowl of mashed potatoes, if it's not put into the fridge a couple of hours after serving it. These starchy dishes are breeding grounds for Bacillus Cereus, a microbe that can cause severe vomiting or disabling diarrhea.”

All Things Mashed Potatoes HERE