1/5 of a metre. 0.2m. 20cm. However you like to write it :)
This is because 3/5 is being divided by 3. So a third of 3/5 is 1/5, just as a third of 3 is 1.
Also, if you change it decimals, you get 0.6m ÷ 3 = 0.2m. You can then change it to cm if you like. 0.2m, 20cm, 1/5 of a metre. They're all the same thing and equally acceptable as an answer.