Third ending won't play

There are a number of issues here. For one thing, there is no repeat sign after the second ending, so it isn't clear where you intend the performance to continue. I guess maybe you wish it to continue with the measure after the third ending, but even though MuseScore does this, it really standard notation. The thing you have notated as a third ending really should be a coda, appended to the end, not placed between the second ending and the passage that follows. I gather you may be working from a printed edition that did it that way, but it isn't advisable, and in any case, as you can see, MuseScore isn't dealing with it.

it's conceivable it might be possible to tweak something to trick MuseScore into playing it right, but I'd recommend instead fixing the notation, so human musicians won't be equally confused.