Flowering question??

Hey my plants have been in flowering for 3 weeks now. One grew 8 inches and the other couple only grew 5 inches is that normal or shouldi have better growth? i heard they are supposed to double in size... Need feedback thanks

How big were they initially? And what strain?

Stretch depends on genetics, too. Generally an Indica isn't going to double in size but they all keep growing for about the first month of flower. A strain which is predominately/purely Sativa might double in size.

I have two og kush and two purple trainwrecks the trainwrecks are growing faster but only 8 iches in 3 weeks and the og only 5 inches. started off at 18 and 15" now 27" and 20" is that normal for 3 weeks of flowering??

They r also n 6 gallon pots