I know you can 'automatically' number a series of vertical cells by selecting
the cells and the clicking on the 'numbering icon' in the menu. Is it
possible to change the formatting of the number sequence?

Example:
Cells A1-A5 automatically number as:
3.1
3.2
3.3
3.4
3.5 or any such numbering scheme.

I tried playing with the settings under 'bullets and numbering' and was able
to create a beginning 3.1 in cell A1, but the remaining cells were numbered
4.1, 5.1, 6.1 etc....