Four Good Measure

No problem for someone who had explored the Labyrinth. Here's one possible solution:

Fill the three-cup vessel to the top. Pour it all into the five-cup vessel.

Fill the three-cup vessel again. Pour all that will fit into the five-cup vessel; this will be only two cups, leaving one cup in the three-cup vessel.

Empty the five-cup vessel back into the bowl. Pour the one remaining cup into the five-cup vessel. Refill the three-cup vessel and add it to the five-cup. You now have four cups in the five-cup.

Drink it all up, and see how easy it was!