Other elegant implementation
rs = RandomSample[Range[Length[input]]];
train = rs[[1 ;; Round[Length[input]*0.7]]];
validation= rs[[Round[Length[input]*0.7] + 1 ;;Round[Length[input]*0.9]]];
test = rs[[Round[Length[input]*.9] + 1 ;;]];
and then use
input[[train]]
input[[validation]]
input[[test]]