I have a cell (B2) that contains "RT3ERIDU"

I wrote a simple formula =IF(ISNUMBER(MID(B2,3,1)),LEFT(B2,3),LEFT(B2,2))

If the third character is a number I want to extract the first 3 characters
otherwise extract the first 2.

This isn't working and I don't understand why.
If I setup another cell with just the MID function then the cell displays
the number 3.

What am I missing here?