If you mean they're in week 3 of veg, and you wonder how much bigger they'll be in week 6, I'd say more than double because the speed of growth is exponential, and that's what I've experienced. For me, it's after week 4 that they grow fast enough to make me notice. By week 6, they're a decent size, maybe around 2' topped.
Yea ^ if you mean week 3 of flower, then around week 3 or 4 they're usually done putting on length. Those first three weeks of flower have the fastest growth in my experience.