whilst there car EMUs are indeed avilable- there's also 3-car 375/377s, most of Scotrail's EMU fleet (314/318/320/334) plus others (313, 507, 508) the Cambridge-London services (both lines) currently run as 4/8/12. Adding a sub-fleet of 3 car units would complicate matters and possibly result in some trains being a carriage shorter than they really need to be.