If your data has a header in Sheet1!A1
and Sheet2!A1
then in Sheet3!A1
you could use (and drag down):
=IF(ROW()<=ROWS(Sheet1!$A$2:$A$4),
Sheet1!A2,
SUBSTITUTE(
INDEX(Sheet2!A:A,
ROW()-ROWS(Sheet1!$A$2:$A$4)+1),
"",""))
If the row number in the new sheet is smaller than or equal to the count of rows in the range of Sheet1
then the result is Sheet1!A2:A4
if the row is greater it'll index Sheet2
column A and will get the result for in that range with the row that equals to the current row number minus the total count of rows of Sheet1!A2:A4
+ 1 to take the header into account.
In Office 365 a simple =HSTACK(Sheet1!A2:A4,Sheet2!A2:A3)
would do.