If you want to explicitly create those subset of data on disk, I think the only way to do something like this without programming is to use StratifiedRemoveFolds as the WEKA filter. You could specify 3 folds to get a 1/3, 1/3, 1/3 split. It doesn’t quite give you 70% though.
Note that the random split mode of the Experimenter performs stratification (unlike the percentage split option in the Explorer or the corresponding mode at the command-line). So, if you just want to get performance statistics, this is a good option.