After 2 weeks in that tiny amount of soil, they will need transplanting ASAP!
There would be hardly any room for root growth and this will stunt your plants severely and is probably why the smallest one has stopped growing.
Transplanting ASAP is the only way to help these plants.
Starting seeds in cups this small, even if filled to the top, which they should be, will never give them the best start in life. Use containers at least twice as big and the seedlings will grow much faster and healthier.
When transplanting them, you could bury them as deep as the cotyledons, (the little rounded leaves) and in time roots will sprout from the buried portion.
Hope this helps,............
Organoman.