Either dump them all out and replant individually, or snip off everything except the heftiest one in each pot. If you can't give more sun, give more light.

The 3 to 5 foot thing is not right. The pic below shows a tomato that is overdue for planting out, and it's less than a foot high. It got planted with as much of it underground as above and grew wicked well.

With a pot the size of the one in your hand, I'd put them in the ground when no more than 8 to 10 inches tall. If it were me, right now, I would carefully dump out the multiple seedlings you currently have, separating them into individuals, and repotting one to a pot. When replanting these, set the plant very low in the pot, covering the stem to up close to the top two leaves - roots will grow out all along that stem. What you never want to have happen with tomatoes is to leave them in a too small pot where the roots wrap around inside the pot too much.