4 Player Board Game

Four players are playing a game involving choosing $1\times1$ squares on a grid of size $3 \times 8$. Each player chooses a random square on the grid, then all players reveal their choices and a token is placed in the center of each of these squares. The probability that the tokens form the vertices of a non-degenerate rectangle can be expressed as $\frac{a}{b}$ where $a$ and $b$ are coprime positive integers. What is the value of $a + b?$

Details and assumptions

Players are allowed to have selected the same squares. There is no restriction on their choices.
A degenerate rectangle has 0 area.
Squares are rectangles.