One of the common requirements for complex table design in adobe forms.
Step 1: Create table.
Step 2: Now suppose you want to merge first and third column vertically, for that you need to split logically second, fourth and fifth column.
Select second cell in body row. and change type of cell to Subform.
Step 3: Now add two Text Object in Subform (Created in Step 2).
Do same steps for other two is well.
i.e. you can also do some border work to looks like them separate.
Now, Visually it looks like first and third column cells are merged.
Step 4: Preview it.
Hope this helps.