three weeks old......you should be fine without raising the humidity, your plants are out of that stage. Typically the humidity domes are for seeds, cloning, etc.... your humidity in the closet is fine now, and will raise as your plants get bigger, but keep in mind that if you raise the humidity, your raising the chances for rot...

but if you do want to raise the humidity of your room, just do what tex said and that will raise it some...