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1.
When two bros decide to add a third bro to their brolatioship. The third bro can move in with the two existing bros in their living space or he can adopt both bros into his own. While the three males may all be completely heterosexual (see totally straight), by this point at least one bro is usually able to willingly identify himself as being bromosexual.

It derives from the french "menage a trios" (meaning household of three) and the english "bro".
Bro 1: You seem like a pretty chill bro. Want to start a brolationship?

Bro 2: I already have one.

Bro 1: Bronage a trois then?

Bro 2: Hellz yeah!
by ILoveYouBro November 23, 2010