Best way to separate train, valid and test sets when doing a Segmentation task

Hello everyone, I’m having a little bit of trouble doing separation of my training, valid, and test sets in a segmentation task. I have the images and labels folders, and I want to 20% of the to be in my validation set and other 20% in my test set, but there isn’t a test_pct function to do this. Does anyone know how to do that split with fastai?