The arithmetic behind the row
A single row of three is width = w1 + w2 + w3 + 2 x gap. The anchored trio is 11 + 16 + 11 inches of frame plus two 2.5 in gaps, so the arrangement is 43 in wide. Centre that on a 72 in wall and the first left edge sits at (72 - 43) / 2 = 14.5 in from the left corner.
Odd numbers give you a middle
Three pieces have a natural centre, which is why an odd count is easier to balance than four. Put the largest or most contrasting piece in the middle and the eye settles there. If all three pieces are the same size, the composition depends entirely on even gaps and a consistent centre line.
When to use the column instead
A vertical stack works where a row cannot: pier walls, the strip beside a staircase, and narrow bathroom or hallway walls. Keep the gap the same as you would horizontally and expect the top frame to sit well above eye level, which is fine as long as the block centre stays near 57 to 60 in.