commas in col

How do I display comma separated values in a column using column A of tbl MMM

Say Col B of tbl NNN = Col C of Tbl MMM

I'm displaying values in tbl NNN & col A from MMM.
I have rows like
xxx yyy 1 zzz
xxx yyy 2 zzz
xxx yyy 3 zzz
but I want
xxx yyy 1,2,3 zzz

What's the SQL (MS SQL Server)?What's the MS SQL Server SQL for making rows

xxx yyy 1 zzz
xxx yyy 2 zzz
xxx yyy 3 zzz

into a row
xxx yyy 1,2,3 zzz