The increased healing from potions is stackable, meaning that you can have (with 5 Dark Seals, assuming you leave 1 item slot for a potion) a total of +125% increased healing from potions. Note that this passive effect only increases healing and not mana restoration.
If you use your three potions (without Secret Stash), you're getting 112.5 health from this item's passive effect. This is 52.5 health more than Doran's Ring and increases its value to 766.25, making it 218.93% gold efficient.
With the Secret Stash mastery you gain 11.25 additional health, which raises The Dark Seal's value to 796.25 ( +30), making it 227.5% gold efficient.
With 5 Dark Seals and Secret Stash, consuming a Health Potion will restore up to 371.25 health, more than twice its original amount.