The flaw is that an omnipotent being by definition can do anything, therefore there’s no way anything can be “necessary” for it. An omnipotent wants something to be so, and it is.
And decades - millennia in fact - of attempts to evade the issue doesn’t change that. The flaws in all those arguments are blatant, they are just ignored by the people making them because otherwise they’d have to admit that their entire belief system is utter nonsense and this whole debate over Christian dogma is on the level of arguing “Can Superman tear his own head off?”
Just more pretentious, and with the difference that people will actually kill other people over it.
But that doesn’t change the fact that the whole tri-omni concept is full of paradoxes, that people ignore that it’s full of paradoxes, and that we very, very blatantly do not live in a world that would be created by anything approaching such an entity. No amount of sophistry, stubborness and evasion changes any of that.