This is the wrong way around. The chicken should be lying on it's front for this method
.... and why wait for a count of 3??
I place the bird on the ground chest down whist holding it's feet behind. I do it in a darkened room so they are calmer. You can stroke them to calm them. I place the broom shank right behind the head, so at the top of the neck, not the bottom. The broom head keeps the shank from applying pressure to the windpipe until I quickly stand on it either side of the head and pull sharply upwards and forwards on the feet so that the neck is being put under the most strain at the point where it has least flexion. Like @OrganicFarmWife I give a second or third pull immediately afterwards just to be sure. Some still flap up to and even longer than a minute afterwards, it seems to vary quite a lot from one individual to another.
I can't say that I am by any means an expert at the technique but this is what I have found works best.
I have started dropping them into a homemade cone after cervical dislocation and severing the head with a sharp knife at the point of the neck break so that they can bleed out a little as they spasm.
So you are pulling opposite to the natural bend in the chickens neck?