Thanks to flexbox the Bootstrap 4 grid has the option of auto layout - columns share equally the available space in a row unless a specific size is assigned.
.col 1 of 3 (33%)
.col 1 of 3 (33%)
.col 1 of 3 (33%)
.col 1 of 1 (100%)
.col 1 of 2 (50%)
.col 1 of 2 (50%)
.col-8 Fixed 8 of 12 (66%), the other columns in the row will split the remaining 4 (33%)