Did you take gravity readings to confirm that the brews were actually done before bottling them up? Also, three weeks isn't all that long, IMO, to go from boil to bottle.
I go 4+ weeks in primary, check with a gravity reading to make SURE the brew is done before bottling/kegging it up. I also taste the gravity sample to make SURE it's actually ready for bottle/keg. Taking the gravity reading and tasting it can save you from what you're encountering. Provided there's not something else in the mix causing your issue.
Out of curiosity, what kind of sugar are you priming with? I've had excellent results using Dememera sugar (aka Turbinado, aka raw cane sugar). You can use a bit less than corn sugar too. Not that it matters. Also, are you weighing the priming sugar? Are you boiling the sugar in a cup, or two, of water before priming?
BTW, unless you ferment in stainless, using 'boil'n hot water' is not a good idea. Going above the listed temperature limits for glass, or plastic, fermenters can lead to a few issues. The least harmful is melting, the most is shattering (hurting YOU in a bad way)...
BTW, Yooper, nice to see the return of your 'fun' avatar... :rockin: