For a generic 1:2:3 mix as you describe, using 5 gal buckets gives a total of 30 gallons of ingredients.
Concrete ingredients "consolodate" as they are mixed...the sand fills in the spaces between the stone, the fine portland fills in the spaces between the sand, the water fills in the remaining micropores.
All said, it takes about 1.5 cu ft of ingredients (not counting water) to get 1 cu ft of concrete. Your 30 gallons of ingredients will consolodate to about 20 gallons of concrete.
In general, plan on about 7.5 US gallons to 1 cu ft, so your 20 gallons equates to about 2.67 cu ft of concrete, or about a tenth of a yard.