I love that you can get multiple usb inputs with the kit… Is it possible to access all three at once?
Yep! I did a show two weeks ago with a MIDI keyboard, a USB Drive and a Web Cam all plugged into the Hypno Kit USB-A ports. Worked like a charm on the most recent Public Beta Firmware. I love the kit for this sort of thing. When it comes to NDI and the Hypno Kit, I usually use WiFi or the Ethernet jack out of convenience.
As for Pi4 vs Pi3 vs CM3 there really are no performance differences. The Pi4 maybe seems a tiny bit more snappy and maybe has less dropped frames, but it is hard to tell.
Will it unlock 1080 or 720 video input?
No