John Chrysostom

Then, having abashed those who were unsound in this respect and subjoining, "I baptized also the house of Stephanas," he again drags down their pride, saying besides, "I know not whether I baptized any other." For by this he signifies that neither did he seek much to enjoy the honor accruing hereby from the multitude, nor did he set about this work for glory's sake.

Homily on 1 Corinthians 3 ↗

Theophylact of Ohrid

"I also baptized the household of Stephanas." That is, all who were in the house of Stephanas. He was a great and most illustrious man in Corinth. "And whether I baptized anyone else, I do not know." For me, he says, it is so unflattering to baptize that I do not even remember whether I baptized anyone else. So how can you take pride in baptism?

Commentary on 1 Corinthians ↗

Thomas Aquinas

Then when he says, I baptized also, he mentions the others he had baptized, lest anything less than the truth appear in his words; hence he adds: I baptized also the household of Stephanas. Then because man's memory is unreliable in regard to particular facts, he adds: Beyond that, I do not know, i.e., do not recall, whether I baptized anyone else.

Commentary on 1 Corinthians ↗