You have some confussion on the syntax in your code. Can you try below code.
class H5Loader(data.Dataset):

    def __init__(self):
        self.hdf5_files = ['1.h5', '2.h5', '3.h5']
        self.data_lst = []
        for ind in self.hdf5_files:
            h5_file = h5py.File(ind)
            data_ = h5_file.get('data')
            self.data_lst.append(data_)
            h5_file.close()