Whether you do 1 plant, or 2, 3, or 4, they will fill your 3x3. With more plants competing for light, they will probably stay smaller, but you'll have the advantage of variety, and gaining more experience in less time. Have you considered starting a new crop a month before harvest? That saves a lot of time. The first 3 weeks they don't grow much, and haven't started to smell, so you can put them anywhere. During the Summer I started 3 on a shelf in my office under a 65w LED board.
With unlimited space, and a NYS legal limit of 3 plants in flower, I've been starting 3 every 6 or 7 weeks, but if the old crop runs 90-100 days the babies get kinda huge. You probably need to space them out more.