On November 5, minor party and independent candidates for U.S. Senate received 3.3% of the total vote cast for U.S. Senate. This does not include write-in votes. Also this only includes the 33 regularly-scheduled Senate elections, not special elections.
Ten of the 33 states with U.S. Senate elections had no minor party or independent candidate on the ballot. Obviously if all 33 states had had minor party or independent candidates, the “other” total would have been significantly higher.
The states with only a Republican and a Democrat on the ballot for U.S. Senate were: California, Massachusetts, Mississippi, New Mexico, North Dakota, Rhode Island, Virginia, Washington, West Virginia, and Wyoming.