So if "passer sur - to pass over", why is "three goats pass over the bridge" not accepted?
It's valid, so I'd report it.
"They cross over the bridge" means there is a bridge and they walk across it. "They cross the bridge" means the same. BUT "Crossing on the bridge" means there is one group coming in one direction and another group going in the other and they pass each other on the bridge.
In this exercise 'sur' belongs to 'passent'. 'trois chèvres passent' means 'three goats are passing', 'trois chèvres passent sur' means 'three goats are crossing' or 'three goats are passing over'.
The sentence could then be either 'Three goats are passing on the bridge' or 'Three goats are crossing the bridge'. But not 'Three goats are crossing on the bridge' because 'sur' is only there once.
Some people are probably struggling with the concept of passer = crossing something, but passer sur (both words together) are key here:
passer sur qch - to pass over sth