It depends on how much real estate you have available. 3 feet is not very wide and the tomatoes I grow (in containers) easily will have a 3' spread unless I remove any lower branching. Which I often do.

If a narrow planting space remains, I'd stay with the onions or lettuce, which do not develop wide spreading root systems like the toms. And the lettuce would appreciate the shade cast by the toms in midsummer. If more room, you could squeeze in some peppers.

Just an FYI, but the notion of companion planting is pretty much an old wives' tale/garden myth. There is virtually NO scientific evidence to support the theory that certain plants grow best together or provide some sort of benefit to their immediate companions.

Onions are actually wonderful for interplanting. The roots don't need a lot of room, and the plants themselves don't cast a lot of shade. I plant rows of onions two feet apart in November, and in the early spring I plant summer veggies between those rows. The onions don't come out until April, and the summer veggies haven't started expanding a lot by then.

Thanks so much for your replies , I will try onions and lettuce. I heard the same about companion plants, I think it mainly root systems competing with each other.

Not on your list but I grow basil with my tomatoes because nothing tastes better together than those two (to me).

tj

thanks, I am going to plant some basil around my tomatoes plants

You could plant lettuce, onions, spinach, bushy peas, or try radishes in the Spring when it is cooler. You eat the cool weather crops before the full heat of Summer (except some onions you can let mature), and then the tomato plants will grow large by mid Summer. That way you use the space before the tomatoes mature.

I like to do basil in with the tomatoes because, though basil likes heat, it doesn't seem to like full sun.

thanks everybody , great help