Watched the whole debate last night where all three were super polite.
Hillary nailed the policy discussions - no contest.
Obama is his own worst enemy in policy. So, steer discussion to policy and let him flounder.
Hillary's core supporters already get that he is weak in many areas and Obama's supporters don't want to hear any crticism, so further attacks on Obama will probably not convince anyone to change vote.
Best bet, focus on core support and get out vote.