There are 15 chairs in a row. Position of P is at the middle. Q is at 12th position counting from the right. How many chairs are in between P and Q?

Let's number the 15 chairs sequentially from left to right: $1,2,3,\dots ,15$.
1. $P$ is right in the middle $⟹$ Position of $P=\frac{15+1}{2}=8$ th from the left.
2. $Q$ is at the 12th position counting from the right.
Position of $Q$ from the left = $(\text{Total chairs}-\text{Position from right})+1=(15-12)+1=4$ th from the left.
3. The chairs strictly between $P$ (position 8) and $Q$ (position 4) are at positions 5, 6, and 7.

$$\text{Number of chairs}=8-4-1=3$$