Rihanna, Leo DiCaprio, Nina Agdal: Three's a Charm?

Leonardo DiCaprio had either the best night of his life or a super awkward encounter with former GF Rihanna and new rumored fling Nina Agdal ... both under the same roof at the same time.

The three partied at Up & Down club in NYC Tuesday night as RiRi looked awesome, paying homage to Nine Inch Nails. Meantime ... Leo had his hands full trying to go incognito with the S.I. cover model.