To solve the problem, we need to identify the pattern used in the triangles. Let’s examine the given numbers in each triangle:

1. In the first triangle, the numbers are 5, 6, and 4, and the middle number is 34. We can express this as: 5 \times 6 + 4 = 34.
2. In the second triangle, the numbers are 9, 2, and 8, and the middle number is 26. This can be expressed as: 9 \times 2 + 8 = 26.
3. In the third triangle, the numbers are 7, 6, and 3. We need to find the middle number which replaces the question mark. Using the identified pattern: 7 \times 6 + 3 = 42 + 3 = 45.

Thus, the number that replaces the question mark is 45.