Try to separate them, be willing to sacrifice a plant or two. I follow the advice to pinch off lower leaves and plant deep.

I would try to separate them. You have to be very gentle, but tomato plants are forgiving :).

Do you need three plants or one? If one, simply snip off the two weaker plants at soil level.

If you want to separate them, let the soil dry out a little...it will be easier to loosen up the roots and do less damage. I haven't bought tomato plants in years, but when I did, I always looked for cell packs that had "bonus" plants.