Please note: This FAQ Article is specifically for the Hemlock theme.
The most common reason Hemlock's full-width grid layout goes from a nice 3-column structure to an awkwardly-spaced 2-column structure is due to a change in font. Sometimes when a user changes the font within grid layout, the font will oddly add an extra pixel or so. These extra pixels can "push" the 3rd post in a row down to a new line.
To resolve this, you can copy & paste the following into Appearance > Customize > Custom CSS:
.sp-grid li {margin-right: 15px;}
@media only screen and (min-width: 768px) and (max-width: 960px) {.sp-grid li:nth-child(3n+3) {margin-right: 15px;}}
@media only screen and (min-width: 480px) and (max-width: 767px) {.sp-grid li:nth-child(3n+3) {margin-right: 15px;}}
If for some reason the above does not work for you, please feel free to open up an new support ticket and let us know.