At the end of the day, he had tested 120 boards, each with three tacks, for a total of 3*120=<<120*3=360>>360 tacks
He had 30 tacks remaining in each of the three cans, for an additional 3*30=<<3*30=90>>90 thumbs tacks.
Therefore, the original total combined number of thumbtacks in the three full cans of thumbtacks was 360+90=<<360+90=450>>450 thumbtacks.
#### 450