Yeah, the only problem I see with doubling up is that you'll also be doubling up on the specialty grains, and I don't think you'll need that. You can definitely add a little more extract to up the gravity, and if you wanted you could add a little more specialty grains, but with almost 1.5lb of crystal in the batch already, I don't think I'd go up to 3lb.
Are the grains crushed already? If not, I'd just plan on saving the grains from one batch for a future batch, then take that 3lb of extract and split it between the other two kits, giving you 4.5lb of extract in each batch. That should up the gravity a little without becoming too cloying from too much crystal. You could even split the hops between the two to help up the IBUs a little, but you'd have to plug that into a recipe to make sure you weren't getting too bitter for the style. I admittedly tend to like less crystal in general, so this is just what I would do.