If it was me, and I do plan on doing this to another palm I have that has 3 plants, I would take the plant out of the pot and place it in a huge tub of water to remove the dirt and loosen the roots. The trick is to try not to break or damage the roots. I know some plants are more sensitive than others, but you might try to move the plants around teasing each one out from the other. It will be a slow process but I think it would work nicely reduce the damage that causes a lot of the problems. Many palms will grow new root systems before continuing to grow if the roots are damage. Be careful.

I think pygmy dates are pretty tough to kill by separating. After asking a similar question, I decided to go out and 'rip' out some very small pygmy shoots from the clusters I had planted in my yard. I had fully expected to kill the small things in the process of pulling them from the ground. As a result, I now have 5 young pygmy dates potted and growing. I imagine if yours are still in a pot, that if you remove the soil, it will be quite easy to separate them.

I'd wait until the start of summer. nothing jump starts growth and rooting like heat.