I've done my share of river rafting as a guide and as an instructor. Those light PVC rafts fron Wal-Mart, et. al. are possibly - and I'm being generous here - suitable for a glass-smooth lake that is shallow enough to stand in when it springs a leak or otherwise dumps you in the water. Also, they typically aren't rigid enough to provide support if you need to torque hard on the paddles (or oars) to accelerate, slow down or turn rapidly.
Even though class 3 rapids aren't generally exceedingly complex, in this kind of raft the are equivalent to class 6 (impassable) in a "real" river raft.
I don't doubt it, those Wal-Mart PVC rafts are probably only suitable for use by kids in a swimming pool or a small smooth-as-glass lake that is shallow enough for you to stand up in if you dump it. Or if you're planning on being within 50 yards of shore the whole time.