Fold in the operands matching:
let processOperator operands operator =
match operator, operands with
| Modulo, [a; b] -> (Convert.ToInt32(a) % Convert.ToInt32(b)).ToString()
| Equals, [a; b] -> (a = b).ToString()
| If, [ a; b; c ] -> (if Convert.ToBoolean(a) then b else c).ToString()
| _ -> failwith "wrong number of operands"
Better yet, if you can, change the datatype to the following.
type Operator =
| Modulo of string * string
| Equals of string * string
| If of string * string * string
Then in the match, you can no longer fail.