Merging cells

I have a spreadsheet where I would like to merge the cells in each row into one cell, excluding the first three columns.
So columns A to C I don’t want to merge.
However, I would like to merge D1 to M1 into one cell in row one and so on, for example.
The problem is that in each row the number of cells that have data in them to be merged will vary.
So I’m asking for a formula that is capable to search each row and merge all data, after column C.
For each merged cell I would like to have some sort of separator, say a comma, to distinguish the merged cells.
I hope that this hasn’t been asked before, or that this hasn’t been too confusing.
Thanks for any help