3 Pdr projectiles are 47mm diameter. It has a Nordenfelt style rotating band (British Design). Kind of a mystery that it has no headstamp. It might have been a dealer sample, to show that any country could have their headstamp on the case, or it left the factory under an overcoat.
Since there isn't a primer, you can look into the case to see if the projectile is fuzed. You can stand the projectile on end and squirt some penetrating oil into the case, so it soaks around the projectile to free it up. If the projectile isn't fuzed, you can use a piece of pipe like an inertial bullet puller, putting the round into the pipe, so the case rim catches on the end of the pipe. Put some padding in the pipe, and with the round pointing down, rap the bottom open end of the pipe on a hard surface. The projectile will gravity/inertia out. It might take a few raps.