Being a 3" frame I assume means that the width of the frame is 3" which means that the frame is going to be 6" longer on each side since we have to take into account the 3" extension on both sides. This means that the dimensions of the frame will be 17" X 21"

The frame has 2 sides that are 17" and 2 that are 21". Add the lengths of all sides together to get the perimeter, which is 17" + 17" + 21" + 21" = 76"