What is an easy way to concatenate a row of data and add a comma between data?

I want to create a line of data in A1 B1 C1 etc. with a comma between each in the created line of text: e.g. A1,B1,C1

(BTW: Prefer the semicolon as your delimiter in sequences. It doesn’t conflict with either of the deciamal separators used in different locales.)
Assuming your version is 5.4 or higher:
If you want empty cells not to contribute to the list: TEXTJOIN(",";1;A1:C1)
If you want delimiters also inserted where empty cells were evaluated: TEXTJOIN(",";0;A1:C1)
There is a help page about TEXTJOIN().

if you’re looking to concatenate text in 2 cells, say, try =cell1 &";" & cell2