Can I use only urad dal to make dosa?
Main ingredient for making a traditional dosa is rice. Rice can't be substituted. Although you can definitely reduce the amount of rice you are using.

From what you've mentioned, you are using 3:1 rice to black lentil ratio. Using 2:1 rice to lentil ratio can also yield you similar batter without compromising the crispiness.

I am assuming that you only use rice and black lentils. You can try adding more types of lentils like Moong (Split Green Gram lentil with skin removed) or Toor Daal.
The point here is to reduce the amount of rice in per serving by adding lentil. I usually follow 4:2:1 rice to black lentil to Moong Daal ratio.

If you completely cut out rice, you can still get similar crepes using substitutes but they won't be authentic dosa. You can try Rava Dosa (Semolina Crepe) or many other substitutes mentioned here