The main thing is to work systematically. Start by putting one of each triplet on one table. Then use them as a guide for the rest of the tables.
For example, consider the picture below. If that is the order around one table, then the alien will need to sit next to a farmer and a surgeon on the second table, and the alien will need to sit next to a diver and an astronaut on the third table.
If you use this method of solving the puzzle, you should find an answer quite quickly.
Have fun!