def get_bl_data(adni_comp, clin_data, scan_data):
    """This function extracts the data from the baseline visit only for each patient.
    
    Supply the three dataframes adni_comp, clin_data, and scan_data as input.
    """
    
    # extract the baseline data only
    adni_bl = adni_comp[adni_comp.EXAMDATE == adni_comp.EXAMDATE_bl]
    clin_bl = clin_data[clin_data.EXAMDATE == clin_data.EXAMDATE_bl]
    scan_bl = scan_data[scan_data.EXAMDATE == scan_data.EXAMDATE_bl]
    
    # return the three dataframes
    return adni_bl, clin_bl, scan_bl