If they're both telling the truth: since B gives maximum disutility, being mugged by both is no worse than being mugged by B. If you think your maximum disutility is X*3^^^3, I think if you run the numbers you should give a fraction X/2 to B, and the rest to A. (or all to B if X>2)

If they might be lying, you should probably ignore them. Or pay B, whose threat is more credible if you don't think your utility function goes as far as 3^^^3 (although, what scale? Maybe a dust speck is 3^^^^3)