I you can speed the grow of the runt. The same effort would get more result from the big one.
My recommendation is to toss out the runt and use bigger pots for the two bigger plants.
Speeding grow during veg is to slowly increase nitrogen until the tips are a little burnt. Supplement with sea weed and amino acid. For me, I water with diluted whole milk or yoghurt once a week for two weeks. 1 part milk - 4 parts water.
It is expensive way of doing it.
I usually add yoghurt to the milk and let sit for a night before feeding to plant. Left too long, there will be cheese kurd that look nasty.