A Tri-City Roundabout occurs when two men and a woman are engaged in wild, funky sex. The woman is on her hands and knees, doggy style, while taking it in the ass from the first man, behind her, and sucking the dick of the second man, in front of her.
The second man blows his load in her mouth; she swallows it just as the first man cums in her ass. Then, she shits out the combined semen of both men, and they eat it. Thus, a Tri-City Roundabout has occurred.
I was watching a porno last night with a Tri-City roundabout; it makes 'Two Girls, 1 Cup' look like Care Bears.