It's likely you're rootbound, have the plants slowed in growth dramatically? The bigger the plant, the more reasons for transplanting IMHO. Those roots are likely wrapped around and around that pot, the only parts of the roots absorbing are the ends which are not as many as could be since they just keep circling. Transplanting also gives fresh soil to the gals, which could lead to less nute issues in the end, IMHO!
Why are you on an 18/6 light schedule, this could induce stretch.
Vegging 3 footers can be a problem, but if you have strong lighting (at least 1000w HPS) and good headroom, go for it! My first was 6'+ at harvest and brought me 10oz of killer bud...not that all results will be similar of course, but it's doable. If headroom is an issue, bend the gal over, she'll be OK and you may be surprised at the outcome!
The strong lighting is so the light penetrates deep into the plant. Less light means airy buds, this is true at the bottom OR the top of the plant!