Reference for Ketubot 121:6

שלש אינה מצעת המטה: הא שארא עבדא ותימא ליה עיילית לך אחריתי לאורחי ופרחי משום דאמר לה נפיש בני ביתא נפיש אורחי ופרחי

If three, she need neither make ready his bed. But her other duties she must perform. Let her say to him, “I brought you a third one to attend for our guests and occasional visitors!” Because he might reply, “The more the number of the household the more the number of guests and occasional visitors.”

Jastrow