I suppose it could come down to having a logjam and then making individual decisions on the various players in question.
De Los Santos got the majority of the starts at 3B toward the end of the season when the 3 real prospects were in AAA. Perhaps the Jays like him and wanted to get a look at him/see if he could improve with some coaching. Seeing as his value is all glove maybe they felt it necessary to keep him on the left side of the infield? And Jimenez, Clement and others were playing SS so 3B is where they put him?
This blocked Orelvis and Parmesan who then switched to 2B and 1B, respectively, which they felt was no big deal because they might move there eventually anyway.
Barger apparently has a cannon for an arm and he had never played OF before. Perhaps they felt it was his best chance to get some reps in RF before turning him into a UT player.
I suppose it isn't a nail in the coffin of any of Orelivs, Barger and Parmesan playing 3B but it certainly doesn't look great when we knew we would need a 3B and instead were playing fringe org guys at the position instead.