The problem seems to be that you need to create a even number out of 3 uneven numbers. Since that is impossible we must look for tricks.
Trick #1: an empty box has the shape of a 0, so we could leave it empty and interpret it as such, then fill the other boxes with 15.
Trick #2: use an additional "+" inside a box: (3+7) + 9 + 11.
Trick #3: use a decimal point: 3.5 + 11.5 + 15
more?