1 out of 3 plants is behind

Hello This is very common. Sometimes there's just a runt of the litter. But usually it's down to plant expression, phenotype etc. The bigger plants may have a more sativa vibe while the shorter of the two might get more indica traits. But many plants really show their stuff in those last few weeks so don't think this small one might not surprise the hell out of you in the end. Happy flowering.

If you treated them all exactly the same and you know for sure nothing stressed this one out more than the others, there is a high chance it's just genetics. Usually breeders try to breed their strains as stable as possible so all plants grown from the seeds are very similar. Other breeders however want to preserve one specific phenotype of plant which results in seeds that don't produce a similar plant but rather very different looking ones with a slight chance of getting a plant that is almost exactly alike the original mother (in nature every plant is unique unless you clone it where you copy the genetics basically) In your case, it's simply a case of a phenotype that takes a bit longer than the other plants. It might still finish by the same time. Just don't worry about it and learn from it, because obviously no plant grown from seed will be an exact duplicate of another. Cheers and sorry for the long text haha ;)

Think of seeds like kids. Siblings have the same genetics as both parents but different traits are dominant, for example you and your sibling may both have your mums eyes but you have your dads nose and she has your mums nose. When you buy a pack of 10 seeds some will be similar to each other and some completely different because they have the same parents but which traits are dominant is individual to the seed. We call the different varieties phenos and each (stable) strain can have an average of 2 or 4 different phenos.