Concatenate a range of cells with a comma

A easy way (if you don't have that many columns) would be to use a literal array and filter out the empty cells with query. Something like this

=join(", ", query({C3:C; D3:D; E3:E; F3:F}, "where Col1 <>''"))

Also see cell A1 in the spreadsheet you shared.

UPDATED: An alternative (and shorter) way would be to use textjoin()

=textjoin(", ", 1, C2:F)