①∵1km/h= 1 3.6 m/s，∴54km/h=54× 1 3.6 m/s=15m/s；②∵1m/s=3.6km/h，∴20m/s=20×3.6km/h=72km/h；③∵1m/s=10-3km/s，∴3×108m/s=3×108×10-3km/s=3×105km/s．故答案为：15；72；3×105km/s．
