If you are driving in hilly terrain, it'll shift out more often too & if you're trying to run/maintain 75 mph with the 3slow in hilly terrain, it'll surely do it.

How does it behave on level ground, doing 75???? If it doesn't do it, I'd suggest it might be the terrain & speed you were trying to maintain.

If it still downshifts on level ground, then I'd suspect something else.

Seeing as how the radiator & tranny cooler, (if you have one) is up front, check both for obstructions, say from bugs, or maybe a piece of plastic, or paper, or mud clogging their cooling fins, that would cause the tranny fluid to run hotter than normal.

If that checks out ok, then as has been suggested, pull the codes & see if any good clues turn up & post all you find.

BTW, did this begin slowly over time, or suddenly after some event????

Just some more thoughts for consideration.