Merging multiple cells into one cell

I know this is easy but I can not seem to make it work. I am trying to merge information from cells A1, A2, and A3 into A4 with the format of A3, A2, A1. Here is my formula =a3&", "&a2&", "&a1. My problem is that a2 does not always contain data so I am getting 2 commas in between A3 and A1 when A2 does not contain data. Hopefully this makes sense but I need to remove the comma and space that A2 would normally occupy if there is no data there. Like merging a First Middle and Last Name with spaces but deleting a space when a middle name is not listed. jrg