I came across an issue where I'm in need of attach multiple bounding boxes to one element, as that functionality is currently not available I'd suggest the following.
Sorry this is not possible, the only workaround would be to create 3 upload zones.
I agree with Ciryk, this is a needed feature.
Agree, need customer to choose either A6, A5, A4 or A3 on one side of the shirt and same for the reverse. Then each size will have its own price. For now i just created duplicated products. HOPE IT WORKS.