How to extract first and third letter of a variable name

I have a variable list named like H1S1, H1S2 , H1S3 and so on. The variables have numeric values. My question is related not to the variable values, rather it is related to the variable names. I want to pick names of these variables from varlist macro in my program and then identify the first and third letter of the variable names and write them a separate macros. Can anyone help me with an example of how to do it.