A: Measure half the width of painting #1.
B: Decide a distance between the paintings.
C: Measure half the width of painting #2.
The distance between the centres of the wallmounts should equal to the sum of A+B+C.
D: Decide a distance between the painting #1 and painting #3.
E: Measure the distance from top of the painting to the bottom of the frame mount according to the scheme below. Add 0.4" (10 mm).
The hole for the wallmount is placed at the measure D+E away from the bottom of painting #1.