Make sure your gutters aren't overflowing and causing excess water to hit the door area. Water is death to wood.
If these frames aren't in direct contact with concrete or earth, or if the door's threshold or garage apron don't collect water, then this happened because the paint and calking wasn't kept up. If the paint cracks, or the calking crumbles, then water gets in and does its cruel business.
If there is nothing else wrong with the ones that are still in good shape then let them dry out, and repaint and re-calk the sill/frame joint. If there is a small amount of rot then remove all of it and replace it with bondo. Since they are Anderson's they are probably expensive to replace.
I would want to replace the whole frame on the bad ones because rarely is the problem as small as you think. If water has been getting in that much then the framing might be rotted too, maybe even the sill. If you have large black ants in the area then pull the whole mess apart and remove all the rotten wood. If you try a piecmeal repair you will never get the weather stripping right or the bottom sweep and you will curse the drafts until you replace it. You should use a lead or membrane roofing flashing under your threshold and keep everything freshly painted and calked to prevent a repeat.
I can't help thinking that something else was wrong with this installation, or else you just let the rot go until the condition caused leaks etc. Not keeping it painted and calked was like not brushing or flossing your teeth. Instead of a cleaning or filling, now you need a major root canal and a new bridge.