Explanation :
Add two matchsticks in right bottom corner of the upper square, which form the complete 1 square and shows two squares joining each other as shown in the below image. So, we get 3 squares.
Matchstick Image : Solved Answer Add 2 sticks to make 3 squares matchstick puzzle
Matchstick Image : Answer Add 2 sticks to make 3 squares matchstick puzzle