Step measure of $$1^{st}$$ Boy$$=63$$cm
Step measure of $$2^{nd}$$ Boy$$=70$$cm
Step measure of $$3^{rd}$$ Boy$$=77$$cm
LCM of $$63, 70, 77$$
LCM$$=2\times 3\times 3\times 5\times 7\times 11=6930$$
Hence, the minimum distance each should cover so that all can cover the distance in complete steps is $$6930$$ cm.
$$2$$ $$|\underline{63, 70, 77}$$
$$3$$ $$|\underline{63, 35, 77}$$
$$3$$ $$|\underline{21, 35, 77}$$
$$5$$ $$|\underline{7, 35, 77}$$
$$7$$ $$|\underline{7, 7, 77}$$
$$11$$ $$|\underline{1, 1, 11}$$
$$1, 1, 1$$