2939 goblins were killed in total, so that means we had 8817 chances for drops between all three of us.
We collected 43 rare engrams, and 8 legendary engrams dropped.
This means we have a general idea of drop rates:
0.488% ( ~ 0.5%) chance of getting a rare engram drop from a mob
0.09 % (~0.1%) chance of getting a legendary engram drop from a random mob