IMO, furr out the 2x3 to turn it into 2x4s and then put a 2x4 in the 24 inch space so you have one 16 inch on center stud and another 8 inch on center.

That way at least half the space will be easy to insulate

The 23 inch wide insulation is like 20 cents more expensive per sqft so depending on size it could be cheaper to create some 16 inch on center studs