Is one 3rd grade engram will be enough?
Enough for all enemies until the end of the game? Certainly not
It means that i need a high lvl of engram which boosting up amount of purified SL?
Huh?
When you defeat enemies, you get (random amount) of soul fragments, of various size too, depending on enemy.
If you ask Memoria to purify an XS (extra-small) soul fragment (the type you get against most enemies in the sewer, which is dungeon level 1, the only available dungeon in chapter 1), you get 1 SL.
But if you have the Healing engram, asking Memoria to purify the same XS soul fragment gives 1.4 SL, rounded down.
Meaning that purifying 5 XS soul fragments gives 5 SL without the Healing engram, but 7 SL with the Healing engram
If, instead of asking Memoria to purify your XS soul fragments, you eat them (meaning, you use them in your items), you get 2 SL per XS soul fragments
Is this clearer?
For larger soul fragments (from small to extra-large), you get more SL, of course...