I remember hearing this joke in high school; it was originally rather racist, so I'm using occupations instead of races in this version. Here it is:
A blacksmith, a shepherd and a plumber have been summoned to an audience with a king. The king's daughter was to be wed, but her suitor must complete 3 tasks before the suitor can take her hand in marriage. The king orders the three men to complete the following tasks: swim across the river, jump over a barbed wire fence, and fuck a cow.
When the three men reach the river, they dive and exert themselves in swimming across to the opposite bank. The plumber and the shepherd make it across, but the blacksmith drowns.
When the remaining two men reach the barbed wire fence, both men attempt to leap over the fence. The plumber clears the fence, but the shepherd gets his scrotum caught on the barbs and is eliminated.
The last remaining man, the plumber, reaches the cow, but during the attempt, the cow bowls the plumber over and tramples him.
As the king waits impatiently, he gives a sigh of relief as the doors to his court open wide to a fanfare for the victor. The victor was the plumber, approaching the king in a disheveled mess; muddied and with hoof-prints all over him. As the king grants the victor his daughter's hand in marriage, the plumber simply replies:
Keep the princess, I want the cow!