If you are getting a hiss, then carbonation is underway - your yeast are doing their jobs.
What you have to understand is that three weeks at 70 degrees is the baseline for normal beers. Higher gravity or lower templs can make this take longer.
Don't open them, don't do anything - just give it more time. If you put the right amount of priming sugar in at bottling, the beer WILL carb up, it just takes time.
- blogging about making my own beer and raising a lot of kids.
Check out the priming sugar calculator
and the beer calorie calculator
Fermenting: Oatmeal toffee stout
Bottled: Enchantress (big Irish red ale), Thundersmoke brown ale (Yorkshire brown), Belgian golden strong ale, imperial nut brown ale