you calculate it based on the expanded volume but the subtract 10% of the volume because it won't expand all the way. Three bales would be a safe bet if your going with the lower ratio 25% 25% 50%.

PS: you will need more compost next season as your beds will settle.

According to "All New Square Foot Gardening" (page 100)
3.9 cubic feet compressed = 8 cubic feet loose.
(Mel adds: These measurements aren't exact but we're not mixing a cake here.")

So, if you are doing 1/3 1/3 1/3, two bales will be more than you need and one bale will not be enough. Using daylily's calculations, 3.8 x 2 - 10% = 6.84cf per bale expanded volume x 2 bales = 13.68cf. Using Mel's/crossstitch calculations, 3.8 = 8cf expanded volume x 2 bales = 16cf.