Two man coverage on transition 3

[rbruno]

End of the game the other night. 17 year old boys. Team A down 3. Team A has the ball in quick transiton. I am the lead in a two man game. Player A-1 comes to the top of the top of the key to stop for the big game tieing 3. The trail is just across the division line and I have two knuckleheads killing each other in front of me as I hustled back with them and am under the hoop.
How do we cover this? He shot an apparent 2 and all the "center officials" in the stands evidently saw his foot slightly on the line. My partner throws his hands up and signals a three and at the same time looks for me for help with the line. As I stated, all I saw was two twin towers in front of me tustling.
What is the correct mechanic to cover this and don't say hire a third official?