Bereishis Rabbah 80:10 (further discussion of the third day).
Sefaria · Bereishis Rabbah 80:10
ביום השלישי בהיותם כואבים — שביום השלישי הכאב גדול.
“On the third day, when they were in pain” — on the third day the pain is greatest.
The third day after circumcision is the point of maximum physical vulnerability.