6,727,084 tons of bombs dropped on Indo China by the US during the Vietnam War. (from Vietnam Veterans Supporting Veterans  )

2,700,000 tons dropped by Allies in European theater
656,400 tons dropped by Allies in Pacific war
US Strategic Bombing Survey, 1945 

2,756,941 tons of bombs dropped on Cambodia. Full-size map  Graph

This only shows the bombing of Cambodia, not Vietnam and Laos.
Source: Bombs Over Cambodia  by Taylor Owen and Ben Kiernan, mapping by  Taylor Owen. Published in Walrus magazine (Canadian).