The order of their preferences is:
u1(a) > u1(b) > u1(c),
u2(b) > u2(c) > u2(a),
u3(c) > u3(a) > u3(b),
The proposal shall be adopted by a majority vote.
The u1 and u2 groups have agreed to vote for proposal b. u3 invites group u1 to openly join a coalition {1, 3}: u1 and u3, choosing a, increase their winnings without fear of any strategic action by u2.