I see that the game supports LAN multiplayer. Is there support for split screen as well?
I´ve heard that only the N64 version has splitscreen.
No, just install the game on 2 PCs (it's up to 8 pcs also)
Is it possible to play LAN with 3 people AND npc´s ?
Yes, 8 pc max, there is no npc possible.
I played with 3 computers long time ago with IPXWRAPPER on win10.
Dreamcast and N64 have splitscreen but the game run at 30fps (and probably worse on N64 for PAL).