I had the exact same problem with our previous golf trip, 5 is the worst number !
What we did:
In each threesome, play a game of "owl" (6 points to share on each hole, 4 for the winner, 2 for the second, 0 for the third)
in the twosome, classic match play
Winner of the threesome gets 2 points, second get 1 point, third 0
Winner of the twosome gets 1 point
Everybody gives its gross stableford score (12 points to share for each course): winner gets 6 points, second 4 points, third 2 points and others zero
That way, each twosome or threesome has to go to the end and everybody has somethin