3 pin bayonet bulbs (BC3) by Eton / MEMLite. BC3 bulbs are low energy only bulbs. They can only be used with special BC3 lamp holder.

BC3 bulbs are similar to, but not compatible with standard bayonet bulbs, these bulbs have 3 pins where none of the pins are directly opposite each other. This prevents to use of regular high energy 2 pin bayonet (BC) bulbs in BC3 lamp holders.

They are often found in newer houses built when building regulations required the use of dedicated low energy bulbs.