has-3-columns-responsive.css

/* Override Gutenbergs asymeteical column margins with the negative margin trick */
.wp-block-columns {
	margin: 0 -20px;
}

.wp-block-columns .wp-block-column {
	margin: 0 0 20px 0;
	padding: 0 20px;
}

/* Override Gutenberg style that kills flex-wrap below 782 */
@media (min-width: 782px) {
	.wp-block-columns {
		flex-wrap: wrap;
	}
}

/* Our own responsive column sizings at various breakpoints */
@media (min-width: 960px) {
	.wp-block-columns.has-3-columns .wp-block-column {
		flex-basis: 50%;
	}
}
@media (min-width: 1080px) {
	.wp-block-columns.has-3-columns .wp-block-column {
		flex-basis: 33.33%;
	}
}

Gutters should probably be a relative unit. I just use 20px to keep it simple.