Models in unit B that are "engaged" (in B2B or within 2" of a friendly model which is) with both A and C can choose to attack either unit, and indeed can split it's attacks between the two if it has more than 1 attack. Models in B which are only engaged with one unit must Attack that unit. (Multiple Combats - Directing Attacks, the last few pages of the Assault Phase rules, no page number sorry, Digi-edition).
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