Combining three names by using formula fied

Combining three names by using formula fied

I have three fields. 1st field is first name, 2nd field is last name and 3rd field is middle name. How can I combine three separate fields into one with an additional character which is a comma (,). For example, 1st field: James 2nd field: Andrews 3rd Fied: Jack. I want an output the looks like this: Andrews, James Jack.

I want to do this because normally when a lead is converted to potential everything gets combined. Except for the middle name.