Hi. My first post!
3 pounds of maple syrup in 13 gallons would add about 8 points to the OG. AVB comes out around 15.8% with an OG of 1.158 and FG of 1.042. BTW, check out this great tool - KotMF: Gravity and Alcohol Calculator
The real attenuation is around 56%. This is around what I got...