When you move into a new dungeon, you need to choose one of three trap sets, which can be purchased for gold, orbs or emeralds. With the first two sets, you also get a pre-made traps placement.
The "Random Traps" set gives you 3 random traps, and you can change any of them by spending orbs if you don't like the ones you've got. Note that you cannot have 3 of the same traps in the random set, and you have to create your own trap placement for the "Random traps" set. When you get them, you have to build your own defence, as there is no standard (automatic) placement in this set.
You can change the trap set in Edit mode.