The boat has to return.
Hus1 rows wife1 across. Hus rows back. Picks up Hus2 and rows across. Hus 2 rows back and picks up wife2 . Wife2 rows back and goes ashore so hus3 and wife3 row across. Either wife1 or wife 2 goes back to fetch wife3.
It's a fantasy. In actual fact hus1 and hus 2 would row across and one go ashore. The other would row back and pick up hus 3. They would both join hus1 on shore and go running off into the bushes jumping for joy.
I could do alternatives.