I think it wouldn't be so much so as weird, you'd just feel insecure or threatened. I don't think its about the "no trust' thing either, its just that you can help but feel somewhat hostile by this other girl/guy even though you completely trust your gf/bf.
I've never had such cases, but I know some friends who used to continuously flirt with other guys while dating her boyfriend, and yes, it did cause a rift. I'm not completely sure about it but from being an objective person outside of the situation, it looks weird and awkward.
Depends to what extent though. As in seriously flirting/constant presence or just good mates hanging out.
But yes, three is usually a (very big) crowd.