To get USB 3 will require new hardware. But, unless you’re using a USB network adapter, you won’t see an increase in download speed. That is a function of your ISP, your connection bandwidth (dial-up, DSL, Cable, Fiber), the server you’re downloading from, your gateway/router (if you have one) and your network adapter. And then there’s your internal (to the computer) bus. The bus is how adapters, memory and CPU all talk to each other. It has a bandwidth and speed rating as well.
Nah. Even USB 2.0 will keep up with broadband.