I would sperate those 3 seedlings in one cup now, before the roots tangle. You can just gently dump the soil out of the cup and fish out each seedling. At this point they should
just have the single taproot and be easy to separate.
The most important thing IMO when growing outdoors is root space. An indoor plant is fine in a small pot but outdoors the heat and sun will dry out a small pot in no time. When the seedlings have grown as wide as the cup they are in, I would transplant into the ground or a larger pot. In general a plant will grow as wide as the pot
you put it in, so the bigger the root mass the bigger the yield. I wouldn't go any smaller than 15-20 gallons of soil per plant.