But how did the artist have any clue as to what the third trial was like?

To me, you answered your own question. The drawing was an artist's depiction. Nothing more. Who's to say anyone has ever even made it passed the first two trials to even attempt the bridge? The bridge is simply camouflaged, requiring "faith" to take the first step. I assume that when Indy made it across, he eliminated some of the illusion by throwing a handful of sand back across the bridge. Once the illusion was gone it made it much easier for Donavan and Elsa to follow.