Some background: Apollo Command Module had an apparently similar arrangement of parachutes. In the case of Apollo CM, two out of three main chutes had to be properly inflated for the landing to not likely cause a significant injury. In case enough main chutes failed, the crew could cause them to be cut away and a reserve chute to be deployed. IIRC speeds under just a reserve chute were similar to speeds under two main chutes.

There are two significant differences between this spacecraft and Apollo CM: Apollo CM landed on water, while New Shepard lands on land and Apollo CM used a crushable piece of hull for final landing shock attenuation (that's why its landing attitude was slightly skewed), while New Shepard seems to use landing thrusters (a' la Soyuz).

They tested that already. They can land with two just fine. There is video on their youtube channel https://youtu.be/xYYTuZCjZcE?t=2m10s where they test this.

They also tested an emergency escape from the booster near maximum dynamic pressure https://youtu.be/ESc_0MgmqOA?t=51s

I would assume there is a backup chute that can be deployed, probably with an altimeter that can auto far if manual deployment fails.