Do you want 50 yard ammo for plinking in the gravel pit? If so, you could use 4895 and all 3 calibers will go bang.
If you want ammo that will be as accurate as possible with reasonable velocities, you need a different powder for each caliber.
7.62x39 needs a fast rifle powder like 4198 or 1680. A compromise powder that would also work ok in a 308 would be H335.
308 needs a medium speed powder, like H335, BLC2, 4895, RL 15 and Varget.
A good compromise powder for 308 and Creed would be RL15 or Varget.
Creed (with 120 gr bullets) would work with Varget. With 140 gr bullets, I prefer something a tad slower, like RL17 or 4350, or N150.