well this is easy but gross .... ur stomaches maximum stretch is 4 liter if u have endless water, drink till u puke ( into the 5 liter bucket) and ull have 4 liters of water
fill up the 3L bucket, pour it into the 5L bucket, then do that again but fill the 5L one perfectly full so theres 1L left in the 3L bucket. then empty the 5L bucket, put the 1L in then another 3L