(defun chunky (strings &optional (chunksize 3))
  (format nil "~{~A~%~}"
          (loop while strings collect
            (reduce #'(lambda (a b) (concatenate 'string a " " b))
                      (loop repeat chunksize append
                        (when strings (list (pop strings))))))))


(chunky '("a" "bb" "c" "d" "e" "f" "g" "h"))
"a bb c
d e f
g h
"