My time as an AA Gate Agt a long time ago, the MD-80s had this issue. The coach seating was 2x3, but the oxygen masks were 3x3, meaning that all lap infants must be on the 2 seat side.
It was bad because a family size of 4 folks (one kid 2+ and one lap) was very common and they really wanted the 3 seats to themselves. But we had to split the family up.
The other narrow-bodys (727, 757) with 3x3 seating had masks 4x4 in every row.