Formula required- A3= 1S; how to make A4=2S, A5=3S

I can't work out a formula that I can copy and paste for 300+ rows (which can be sorted ascending or descending.
I have several spreadsheets that each have reference numbers starting at 1.

I may have to join the sheets and keep the reference different by adding a letter as well. Thus I need a formula that will augment the number by 1 but will also have a letter to identify from which sheet it has come. which letter is used is not important.

Cell A 3 must be 1A
Formula required to make
A4 = 2A
A5 = 3A etc.

many thanks

Thank you OllyXLS - just what I wanted. Apart from not knowing the ROW() function, I was trying "+" and not the "&".

Many thanks - I knew it had to be simple!!