Was it because the fourth chair 'belonged' where it was, and removing it created an imbalance in the overall area, kind of like a man with a missing leg? if so, I can almost understand it, in a 'well that just doesn't look right at all' sense. Here you have 3 sides of a table being used, and the fourth just...there. The only thing I could suggest would be if you got the other 2 diners to move so that each occupied a third of the4 table, rather than a fourth. That way the table is rebalanced. Awkward to ask everyone to do that, but better than a meltdown.
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Everything would be better if you were in charge.