I did exactly that last spring.
I'm so sick of salsa.
My small garden only takes up an area roughly 5' x 12'.
A tomato plant will take up a good 2' x 2' area, 5' high . The jalapeno about the same but it shouldn't be as tall. Bell type peppers are shorter plants and can be contained in a small tomato cage if you're worried about tipping. My peppers rarely go 3' high. Onions can get very large in a 1' square but many people run them very close together and let them compete for their own space.
My suggestion is that you plant your tomato on the North, your peppers West and onions on the South. If you arrange them any other way, the tomato plants will be tall enough to starve the other plants from sunlight. To cage the tomato plants, I have used the large cages from the rural farm suppliers but the best plants grow in cages made from concrete reinforcement mesh. The 6" squares are just right. As the plant grows, I weave the ends of the plant back and forth between the mesh and it's amazing how fast they grow. I clean a few fish every year and I'm always amazed at how strong the scrap fertilizes my plants. Just dig a hole between plants about 8" deep and dump in a half pound of fish heads lol
Anyway, 2 tomato plants to a box, 2 jalapenos in one box, 3 (or 4) bell peppers in the other and onions around the South and maybe West edges. The next concern will be keeping those 4 to 5' tall tomato cages upright. I lost one last year to the wind from a fast moving afternoon thunderstorm. I'll be building my cages this year with 6' fence posts and 10' conduit laterals. Some fence wire or concrete floor mesh...
This is an old pic of my little garden spot from a couple of years ago