Please provide provide your full system specs.
To answer your questions,
I wanted to know if I can install windows 10 on a usb 3.0 port
It can vary for a lot of hardware and which era of hardware we're talking about. There are a lot of chipset hardware that require a base driver to become functional while some hardware require a stand alone driver to operate so in the even of the latter installation of an operating system off a bootable installer you'll need to plug into an available USB2.0 port instead of a USB3.0 port. Thus my inquiry into your systems specs to reveal your chipset and if drivers were necessary.
do i need internet to perform a clean install of windows?
To be specific, are we looking at a Windows 10 installation using the free upgrade path or do you have a 25 digit activation key? In either case, yes, a
clean install is advised.