downright impossible
と flat out impossible
と straight up impossible はどう違いますか？説明が難しい場合は、例文を教えて下さい。

They are almost the same and usually you just have to learn which to use when.
Downright means completely but generally saved for something negative or bad.
Ex:
His behavior is a downright disgrace. It’s downright impossible to jump over that fence
Flat out means completely as well but it used when people mean “without hesitation” or “clear cut”
Ex.
That’s a flat out lie
They flat out left the country.