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.