I've got a very simple problem, but not so trivial.
I need to split a dataset (input and output data) in three part randomly: training set 70%, validation set 20% and is test set 10%.
First of all this splitting have to be random, but reproducible, so I set SeedRandom.
So I produce this simple code to split in two part (70% and 30%), but I've some problem to split in three part (70%, 20% and 10%).

    SeedRandom[1234];
    index = RandomSample[Range[Length[input]], IntegerPart[Length[input]*0.3]];
    indexp = Partition[index, 1];
    inputv = {};
    outputv = {};
    (*Validation TEST*)
    inputv = Table[Append[inputv, input[[i]]], {i, index}];
    outputv = Table[Append[outputv, output[[i]]], {i, index}];
    (*Test TEST*)
    inputt = Delete[input, indexp];
    outputt = Delete[output, indexp];

    inputv = Flatten[inputv, 1];
    outputv = Flatten[outputv];

Could you help me?
Thanks...