MTG Salvation

Suppose I have Jace, Vryn's prodigy and 3 cards in the graveyard. I cast Radiant Flames hold priority loot with Jace. Would my Jace flip with the Radiant Flames I casted as my 4th card in the yard and Jace loot trigger as my 5th?

The spells on the stack are exactly there - on the stack. They (normally) go to the graveyard as a final part of their resolution. The answer is no, it would not flip.

In this case, Jace's ability will go on the stack above Radiant Flames (C.R. 405.2) and get to resolve before Radiant Flames (C.R. 116.4). Thus, if all players pass in a row right after that, Jace's ability will resolve (C.R. 116.4): you will draw a card and discard a card, and since only four cards will then be in your graveyard, Jace isn't exiled, etc. (C.R. 608.2c)

Note that you don't "flip" Jace if its ability's criteria are met (a term used only for a flip card permanent [review C.R. 709]), but rather exile Jace and return it to the battlefield transformed under its owner's control.

I recall I had a similar incident involving fetch lands:

Where I wanted the top card, 3 cards in the bin, I fetch, hold priority loot with Jace, the judge said in this case, the fetch land would be in the graveyard when Jace loot resolves. Is this correct?

Yes, that is correct. Because sacrificing the fetch land is a cost and thus is paid when the ability is activated already, not when it resolves. So by the time you can activate Jace, the land is already in the graveyard.

Thanks so much