The car park (B) is next to entrance one.
Good waterproof footwear and rain jacket recommended. A headlamp is essential because you have to use both hands. You'll get very muddy.
After about 300m there's a hip-deep pool blocking your way but you can get around it on the left side through ankle-deep water. Take your shoes off and be careful it can be very slippery. After that you'll have to wade through another 20 cm deep pool with a ladder in the middle. If you have proper equipment for abseiling you can turn right at the ladder and follow the underground river. All others just keep following the marked path to entrance three/exit.