def subset_by_dataset(df):
    """
    Returns tumor, normal, and GTEx dataframe subsets

    :param pd.Dataframe df: Dataframe containing a "dataset" and "tumor" column
    :return: One dataframe per dataset
    :rtype: tuple(pd.Dataframe, pd.Dataframe, pd.Dataframe)
    """
    tumor = df[df.label == 'tcga-tumor']
    normal = df[df.label == 'tcga-normal']
    gtex = df[(df.label == 'gtex')]
    return tumor, normal, gtex