USB 3.0 is totally dependent on the mobo, not the CPU. With that mobo, you're golden. No worries, you're good to go.
The "shopkeeper" was probably getting USB 3.0 confused with PCIe 3.0. The i3 can't do PCIe 3.0 because that's a CPU AND mobo level thing.
It's a good thing you came here. Some guys that work in computer shops don't really know as much as they'd like to think they do, lol.