法華義疏 6.20

6.20

蓋是總相稱為諸子，非是門外三人名為諸子。故後合譬釋云：若見無量億千眾生出三界苦得涅槃樂，此但明二乘人出三界得涅槃樂，不說菩薩，則知不必皆是三子索也。

This is merely a general designation referring to them as 'the sons'; it does not mean that three persons outside the gate are called 'the sons'. Therefore, the subsequent convergence of the parable explains: 'If he sees immeasurable hundreds of thousands of myriads of living beings exiting the suffering of the three realms and attaining the bliss of nirvāṇa'—this only clarifies that persons of the two vehicles exit the three realms and attain the bliss of nirvāṇa, without mentioning bodhisattvas; thus we know it is not necessarily three sons who all request.