It's a wild vid, knowing what we know now ... it shows all 3 of 'em posing for pics together in a hot tub before Raquel says these photos must stay on Tom's phone, and can't be shared with anyone.

Ariana's reaction is pretty eye-opening -- "It looks like we're in a throuple." 👀

Tom and Raquel didn't have any visible reaction after she said that, but the timing of this scene is pretty shocking. Bravo aired the previously deleted clip ... which was shot last August in Mexico, where they were all attending Scheana Shay's wedding.

Twitter's having a field day with the clip -- some point out he couldn't keep his eyes off Raquel in the hot tub, while another called it "Beyond gross."

As we all know, Ariana later found out her words had some truth. We broke the story ... Tom and Ariana broke up after she discovered sexual texts between Tom and Raquel ... who'd been using code names on each other's phones to hide their affair.

We spoke to Tom this week, and he said he's happy Ariana's moving on ... referring to her hookup with a new guy at Coachella.