You do not need to mention their nationality or occupation, or anything specific about them, if nothing in the joke has anything to do with stereotypes of those specific nationalities or occupations.
3 people were stranded on an island (4 is too many, people will get bored at that point). Eventually they came upon a tribe of savages who captured them and demanded they venture out into the woods and each grab a round object. They were confused, but did as they were told because they didn't want to get killed or eaten or whatever.
The first guy comes back with a (start with a really small object first) peanut/grape/etc. The chief of the tribe/village elder says, "Good work, now take that peanut and stuff it up your @$$." The first guy was confused but did as he was told.
The second guy came back with a (use a fairly big object at this point since there are only 3 people) peach/apple/orange/apricot/grapefruit. The village elder told him to put the fruit into his butt, and after struggling for about 20 or 30 minutes he finally got it in, and breathed out a sigh of relief.
Then the second guy looks to the first guy, and they both bust out laughing suddenly. The village chief says, "What's so funny?" One of the guys says, "Well, that third guy, the reason he's taking so long is because he's trying to dig a cactus out of the ground."
(If not a cactus, carrying a huge watermelon, or picking a durian out of a tree might work also. A lot of westerners don't know what a durian is though probably.)
