The h3s in monobook are bolder than and subsequently can appear a bit bigger than h2s, despite being less important than h2s.
Headers should appear with decreasing apparent importance according to which header they are.
T65844: h3 should not be more prominent than h2 headings
T71998: Vector: h3 should not appear as heavier weight than h2
T72004: h4, h5, and h6 headers should not have the same styling
T73240: Re-evaluate serif font stack for headers