Each of the overlapping areas contributes to the area of exactly two squares. So the total area of the three squares is equal to the area of the non-overlapping parts of the squares plus twice the total of the three overlapping areas, i.e. $(117 + 2(2 + 5 + 8))\;\mathrm{cm}^2 = (117 + 30)\;\mathrm{cm}^2 = 147\;\mathrm{cm}^2$.

So the area of each square is $(147 \div 3)\;\mathrm{cm}^2 = 49\;\mathrm{cm}^2$. Therefore the length of the side of each square is $7\;\mathrm{cm}$.