Supposing a simplistic chair (stool) with four legs,
If this question was asked in mechanics, the answer would be simply supported.
But in design, we should fix three legs in y direction and one leg in all directions.
That is because the fixed leg acts as reference for the movement of the other three legs. In other words, we are restricting the velocity of the chair in all three directions, ie giving boundary conditions in all three directions.
Then we get the solution as below.
Note that the left leg(as shown in figure 1) is the fourth leg.