How can a recumbent tadpole trike brake three wheels with only two levers?
Most are set up with one lever that works two brakes at the same time. There are lots of different styles, such as this one for example.



You can see that it has 2 ferrules instead of 1, most commonly used on dual disc brakes. There are other models that differ somewhat visually but the idea is the same. Many Trike levers also have a push button locking mechanism that functions like a parking brake so that it cant roll while engaged.