Hi
@obeee,
It really looks like sunburn. For that, I won't do anything special. She will recover and might end up staying smaller than the other 2.
However, 3 plants in a small pot is not going to work. You need to separate them quickly and put them in larger pots. Outdoors, the stretch could be more significant. Run to your grow shop and buy fabric pots of at least 15 liters or more. These autoflowering plants grow very fast, and if you stress them, they will stay small. If you prune, damage the roots, or uproot them, they will need time to recover. The best thing is to sow them in their final pots.
M.