Yeah, that's the problem with most of the small-fruited and cherry types of tomatoes. They are what are known as "indeterminate" which just means that they will keep on growing and producing as long as the weather holds or as long as you can grow them. (take them into a greenhouse, you can keep them producing all winter with enough warmth and light)
The type of tomato that stays smaller is known as "determinate" but the problem with this type is that after it stops growing at a nice height for a container plant it then sets fruit and it all ripens at once, then there's no more. Indeterminate tomatoes continue to produce as long as the weather is favorable.
I think your solution is to grow your favorite type of grape tomato, and pinch or prune the plant once it gets close to the biggest size you can handle. If you want a 3ft plant, start when they're 2ft. tall to pinch off the growing tips of the branches. This will make the plant smaller and more bushy, less like a vine and more like a bush. It might delay the first setting of fruit, though as well. Btw a 5gal. container is about the smallest you should use for any tomato. They do get top-heavy!
Tomatoes naturally grow as vines, and they can get huge. They also can sprawl and hang, so another solution if you have the option is to hang the container up and let the plant sprawl downwards. Or, say if you have a deck or balcony with a railing, grow the plant out over the railing and let it cascade. Might be a challenge to pick the fruit, but it will look pretty.
As far as your 6ft. plants right now, you can try pruning, but it might take a while to set more blossoms, and we're getting down to shorter days here. Maybe just pot it up into something large and keep it going for now?
If you would complete your profile, with your location it will help us to help you in future. City/state/country is needed, just a zone doesn't help much because it only tells us how cold you get in the average winter. We need to know more about your growing conditions in the growing season.
