my trailer arm is 2x3 angle iron...how can I mount this on it?

Yes but you will need to make the angle iron 4 sided so the mount will hold otherwise it will come loose. A couple o ways to do this 1. have a welder weld a similar piece or angle iron on the opposite side on the 2X3 angle 2. just put a piece o 4 side iron under it. See photo in my review