Soemthing like this?
def __init__(self, x, transform=None):
        self.data = [(el, 'none') for el in x]
        self.data.extend([(el, 'augm1') for el in x])
        self.data.extend([(el, 'augm2') for el in x])
        self.data.extend([(el, 'augm3') for el in x])
        self.transform = transform

and then
if self.data[i][1] == 'augm1' and self.transform:
            img = Image.open(self.data[i][0])
            img = self.transform(img)
if self.data[i][2] == 'augm2' and self.transform:
            img = Image.open(self.data[i][0])
            img = self.transform(img)
if self.data[i][3] == 'augm3' and self.transform:
            img = Image.open(self.data[i][0])
            img = self.transform(img)