To get the "latest" USB device drivers, you'll need to either have a Software Choice subscription, or eCS. However there are some older "free" drivers which IBM released that may work for you.
And download the "USB Basic Controller Support" - the latest "Free" driver.
Also further down on that page is the USB Keyboard driver. Grab the "free" driver too. (unless you have a SWC subscription, in which case grab the latest driver)
As I recall, in order for USB to work properly under Warp 3, you'll also need the latest fixpak - which for Warp Server 4 is Fixpak 43. The US english version can be found:
You may also need the Device Driver fixpak (but I'm not 100% sure if this is needed for USB stuff, because I believe the USB stuff was never incorporated into the DD fixpak), at: