Is 287mm smaller than 3dm?

Yes.

Convert to same units than compare

1 mm = 1/1000 m

1 dm = 1/10 m = (100×1)/(100×10) m = 100 × 1/1000 m = 100 mm

→ 3 dm = 3 × 100 mm = 300 mm

287 < 300 → 287 mm < 300 mm → 287 mm < 3 dm