Count numbers using digits . Build digit by digit, tracking tightness. Position (hundreds): if tight, try and 3 (but not 5, since ). If , stay tight. If , become loose. Position (tens): if tight (started with ), try (becomes loose, since ). If loose, try all: , , . Position (units): if tight, try (since ). If loose, try all. Count all valid paths.
Answer: numbers like , , , 131, ..., , , that are .