To determine how many pieces were not sold, follow these steps:

1. Calculate the total number of pieces: Each pie is cut into eighths, and there are 3 pies.$$\text{Total pieces}=3\text{ pies}\times 8\text{ pieces per pie}=24\text{ pieces}$$
2. Determine how many pieces were sold: Three-quarters of all pieces were sold.Pieces sold=34×24=18 pieces\text{Pieces sold} = \frac{3}{4} \times 24 = 18 \text{ pieces}Pieces sold=43​×24=18 pieces
3. Calculate how many pieces were not sold: Subtract the number of pieces sold from the total number of pieces.$$\text{Pieces not sold}=24-18=6\text{ pieces}$$

Therefore, 6 pieces were not sold.