Trying to set a rule that if a cell is blank, to pull in the number from another cell in the record - help! :)

Trying to set a rule that if a cell is blank, to pull in the number from another cell in the record - help! :)

Hi all,

I am currently mapping out daily sales of different products on an hourly basis to start to get a feel of sales velocity. 

This data is recorded on an hourly basis per product, under ascending fields named stock_remaining_1, stock_remaining_2, stock_remaining_3 etc...

I am looking for help with a formula or rule that will state the following:

if stock_remaining_3 is blank, then add the number found in stock_remaining_2

Any ideas would be super helpful - I don't mind if this is a rule added to the field itself, or if I have to create a formula field to reflect this if easier.

Many thanks!

Gareth