What sort of beers do you drink and what IBUs are you after? I still do 1 gallon batches with new hops, I don't think you are going to get 3x 5L brews out of 50g. A high alpha US hop might cut it, 50g of Centennial would give you about 32IBUs per brew, but even though I'm not a member of hopheads anonymous yet you'd want about 40IBU and more late hopping with the US to get a good taste. I would buy 50g of 3 different hops and every 4th brew use up the remainder. First Gold, Perle and Amarillo would be my three.