I am no expert but if there are 3 plants in that container they should probably each have their own container. It might be one with suckers coming up. Either way, yes, get it/them in a much larger container. I'd say something closer to a 10-gallon. If there are more than one it might tame some patience to untangle roots but it can be done. Put a tarp or something down and lay your container on its side and try to shake the whole thing loose. You might need to run something around the perimeter to help loosen. Try not to pull by the plants as it could damage them. Once transplanted, water well but then don't water again until the soil is dry at least 1" down. Good luck!

Roma is a determinate tomato with compact size, so they are getting near to full height.

Agree that they'd be happier if individually potted, but I'd worry about doing damage... I'm guessing those plant leaves are entangled, as well as the roots. To pot it up with the least damage, you could just slide the whole thing out of the current pot and into a larger one. Or if separating, instead of untangling roots, you can just use a knife and slice straight down between the plants. Sometimes a clean cut is better than messing around detangling. :) Like edie said, use a knife to run around the edge between the pot and soil to make sure it's loosened, then tip the pot onto its side on a tarp/plastic sheet. Try banging the bottom of the pot with your hand to get the roots to release... and gently try to wiggle it all loose. You can leave it in the pot it's in and hope for the best. I've kept excess tomatoes in a one-gallon pot and even they produced fruit. :p Definitely plan on keeping it well-watered and feed it with a weak fertilizer regularly.

You should be able to gently separate the roots and leaves. And you should get separate pots for each one. 5 gal MINIMUM! They probably will get knocked back some when you do this, but they'll bounce back. Tomatoes are tough!