Step 1: Analyze the pieces.
In image P, we are given 8 triangular pieces that form part of a puzzle. To solve the puzzle, we need to arrange the pieces in a way that forms a correct and identifiable number. By carefully observing the pieces, we notice that when they are arranged correctly, they form the digits 7, 4, and 5.
Step 2: Arranging the pieces.
After rearranging the pieces based on their shapes and markings, we find that the pieces form the three-digit number 745.
Step 3: Conclusion.
Therefore, the three-digit number formed when the pieces are arranged correctly is \( \boxed{745} \).