Pulse or blend the bananas till you get a smooth puree. Add this to the wheat flour along with sugar and mix well.
Add the milk slowly to make a dropping consistency of the batter. Let it sit for 10 mins.
Get a tawa heated up, make sure it is seasoned well if you are using an iron griddle. Else nonstick pan works best for this.
With this batter, you got to be really quick and most case, pouring this as the regular dosa will not work.
So you will have to pour a ladleful as a circle and quickly spread it as dosa. Sprinkle oil/ghee on the sides and cook on both sides.