The basic procedure is:
1) convert your number to a left-justified character string with the LEFT and PUT functions
2) extract the first3 characters from the left-justified character string with the SUBSTR function
3) if you need the first 3 characters as character, then you're done, But, if you need the first 3 characaters as a number, then you need to convert from a character string to a number using the INPUT function.
charvar = left(put(num, 30.)); /* #1 */
first3_char = substr(charvar,1,3); /* #2 */
first3_num = input(first3_char,3.); /* #3 */
You could have done everything in one assignment statement, but I find it useful to break down each step in the transformation, so it's easier to understand.