In a concatenation you can use
="line one" & char(10) & "line two" & char(10) & "line three"
You will have to turn on cell wrap for such cells
Format, cells, aligment, cell wrap
Failure to turn on cell wrap will show the square printer's bullets
associated with unprintable characters.
If you are just typing text then use At+Enter for the new line
and Excel will turn on cell wrap for you.
In VBA you would use CHR(10) or vbLF