Reza Farahan is spilling some seriously wild tea ... revealing his past fling with 'Biggest Loser' star Bob Harper turned into a friends-with-benefits situation and eventually, a threesome involving Harper's husband.

The "Shahs of Sunset" alum drops the bombshell in 'Memoirs of a Gay Shah,' saying he and Bob first met while working in the fitness world and quickly became "inseparable." Their close friendship soon crossed the line into something physical ... but Reza says it never became the relationship he actually wanted.

Reza -- who is married to Adam Neely -- says he was looking for something deeper but got the sense Bob wasn't interested in anything serious, so he didn't push it and kept things casual.

Things got more complicated when Bob later married a man named Scott. Rather than step away, Reza says he became close with both of them and claims the dynamic ultimately turned into a threesome.

Reza describes the situation as "bittersweet," noting he had no ill will toward Scott, who has since passed away. Despite everything, he says he and Bob have remained friends ... even revealing he recently helped sell Bob's house. Bob has also acknowledged their past, previously admitting on "Watch What Happens Live" the two had "a thing," calling it a "tawdry little" situation.

For Reza, though, the experience wasn't just messy ... it was a wake-up call. He says it made him realize he didn't want to be anyone's second choice and pushed him to seek a loyal, committed relationship.