What's the difference between 1-800, 1-877, and 1-888 phone numbers?

I know they are toll free, but why 3 different numbers?What's the difference between 1-800, 1-877, and 1-888 phone numbers?
I think it is for different regions from where the company started or they wanted to have a phone number, but it was taken and they had to use 1-800, 1-877, or 1-888 to get the phone number.