From https://www.britishrallychampionship...registrations/

“… the British Rally Championship topflight for Rally2 and Rally3 machinery…”

However, from https://www.britishrallychampionship...egulations.pdf

4.1 CLASSES OF CARS
…
BRC3 | Rally 3…

So in the marketing spiel we’re led to believe there is no specific class for the Rally3s and if anyone were to enter the BRC with one they’d be lumped in to the overall standings only. But the official document makes reference to a BRC3 class for Rally3s, so perhaps they do hope/expect that some competitors will enter in a Rally3 for class honours…