Why are my new switches tripping the circuit breaker sometimes?
That sounds like a short to me, and a potential fire or electrocution hazard. Leave the breaker off while you fix this. With a 3-way light switch, you'll have two possible paths between the switches (which is why there are 3 screws + ground, 1 line/load, and 2 switched). My guess is that one of the two switched wires is grounding itself. Remove the switch that you worked on, and see if the ground wires are coming into contact with any other wires.