Digit Summing Function
By the way, the fastest way to see if a number is divisible by three is to use (zero? (remainder n 3)). If you want to do the digit summing for fun, fine, but it will make your code slower.

So, one way to sum the digits is to extract digits directly during the summing process, rather than building a list of digits first.

(define (digisum n)
  (let loop ((sum 0) (n n))
    (define-values (q r) (quotient/remainder n 10))
    (if (zero? n)
        sum
        (loop (+ sum r) q))))


Alternatively, you can use a for comprehension for this:

(define (in-digits n (base 10))
  (make-do-sequence
   (thunk (values (curryr remainder base)
                  (curryr quotient base)
                  n
                  positive?
                  #f #f))))

(define (digisum n)
  (for/sum ((digit (in-digits n)))
    digit))