To merge cells vertically and keep all data in Microsoft Excel, follow these steps:

1. Select the cells you want to merge. In this case, select cells A1, A2, and A3.
2. Right-click on the selected cells and click on "Format Cells" from the drop-down menu.
3. In the Format Cells dialog box, click on the Alignment tab.
4. Under the Text Control section, check the box next to "Wrap text" and click OK.
5. Select the merged cells, right-click and select "Format Cells" again.
6. In the Format Cells dialog box, click on the Alignment tab.
7. Under the Horizontal section, select "Center Across Selection" from the drop-down menu.
8. Click OK.

This will merge the cells vertically and keep all the data. The text will be wrapped within the merged cell and centered across the selection. If you want to add a line break between the original cell contents, you can use the function.

To add a line break between the original cell contents:

1. In a new cell, type =CONCATENATE(A1,CHAR(10),A2,CHAR(10),A3)
2. Press Enter.

This will concatenate the contents of cells A1, A2, and A3 with a line break between each cell's contents. You can then copy and paste the concatenated text into the merged cell.