Yes, but she didn't accidentally say three, I don't think. She meant three, but perhaps mistook the two legs in the back for wheels.
(very paraphrased, but as I recall it...)
Meredith: How many wheels are usually on a wheelbarrow? One, two, three, four.
Contestant: ...C, three. Final answer.
Meredith: Oooh, no. It's not. It's one.
Contestant: What? Oh, s***! *covers mouth*