I'd recommend giving the Sarracenia LOTS of water (they don't really mind being flooded as I've seen) and put it in an area where it wont get excessive sun.
As for the Nep: Hahaha! I have one that I NEVER water, it's just in a closed container (used to have pretzels inside; they were yum). I've forgotten about it for many weeks at a time and it's perfectly fine. I'm not saying that yours will be such a smashing success as mine has a very coarse mix (sphagnum moss and orchid bark) so that probably allows the moisture to reach the roots better than in say a largely peat based medium, but I'd give it a shot. I've never had a problem with mold or anything despite the warm, stagnant conditions. So what I'd advise to you is to put your plant's container in to a simple dish that will hold some water (dish size dependant on pot size, of course,) and then put lots of water in the bottom of the container (ie, not in the dish).
As for the Drosera, I know that in propagation and in the wild, they are often submerged, so give it a shot! Probably not enough to keep the plant submerged for all 3 weeks, but enough so that the water table will fall and by the time you get back it wont be bone dry (not that you can really know how much water that should be...) If you're worried about evaporation, put the plant in a container and cover it halfways (that way you still get a little airflow for when the water's receded.
Just what I'd do if I couldn't get someone to look after my plants, you're welcome to try what you think sounds bests. Oh, and if you're really worried about the Nep, you can give it some fungicide. Like everyone said, 3 weeks is a long time and it'd really be best to get someone to water them if you can, but sometimes you just gotta roll with the blows.