Additionally to editing the weird file, I request that the weird file contain the before-both version ("conflictStle" set to "diff3").
A thing that bothered me in all (I think) merge tools I've tried is the conflation between the final state I want and something else (i.e. you do something that feels like editing one of the three "inputs" to make it look like the intended state; then the diffs between the various columns change their meaning as soon as I start actually resolving conflicts, so I need to keep track of the state I'm in in my head.