Being Mary Jane actress Union, 42, was originally recruited to participate in the competition against the late-night host, but Duhamel, also 42, couldn't resist getting in on the fun.
"Gabby and I are about to go head-to-head in a random object shootout," Fallon, 40, said. "We're going to take turns shooting these random objects into that basketball hoop behind us... Josh Duhamel is the ringer, okay? We each get one JD each."
After Fallon explained that Battle Creek star Duhamel could be brought in to take a guest shot for each competitor, the actor chimed in, "You just added this last minute. I wasn't going to... I was supposed to go backstage but I wanted to stay, so thank you! Thank you for including me!"
Fallon and Union, with Duhamel at their disposal, then faced off, trying to make baskets with a bevy of bizarre objects.
In lieu of basketballs they shot Union's husband Dwyane Wade's sneaker, which was full of Twizzlers, an Etch A Sketch reading "Jimmy and Gabby," a doll bathtub filled with super bouncy balls, each of their go-to meals (a "healthy salad" for Union and a "healthy-sized turkey sandwich" for Fallon), and the so-called "money ball," a cone of the newly released The Tonight Dough Ben & Jerry's ice cream.
See which star came out on top in the video above!