You can grow a tomato in a 3 gallon pot, and have it terribly stunted if an indeterminate type, but you'll still get "some" tomatoes. Use double the # of 3 gal. pots if that's what you have. 5 gallon pots, you'll get more, and so on with larger pots. 20 gallons is usually about all you'll need unless you're feeding them quite a bit of fertilizer and have a long season. Determinate types usually don't need over about 8 gallons to get diminishing returns from a larger size pot, but again depends on how well you treat them.
The problem I have with pots is, I'll get some August storm with strong wind right about the time I have 10 lbs of fruit towards the top and it'll take one or more plants down. I've had wind break stakes, pull out of the ground, or even tip the whole pot over.
Personally, I wouldn't even bother with less than 5 gallons.
One option to get around the weed killer is read the instructions for it. Most have a limited amount of time they're effective so if you can wait that out, say start the plants in modest little pots, then transfer them into the ground once the weed killer has dissipated. Tomatoes transplant great, bury them up to the first set of leaves (deeper is better).