If said in another way, rank 3 spells can be used with more than one life seed because of their increased complexity, while rank 2 spells can only be used with one life seed.
So if a cultivator at the second mortal coil manages to learn and use a rank 3 spell, they won't be able to use it to its full capabilities because they only have one life seed to use it.
So only a cultivator at the third mortal coil can bring out the full benefits of learning the more difficult rank 3 spell. Cultivators at the second mortal coil, no matter how intelligent and special they are, are better off just sticking with rank 2 spells.
The opposite is the case too. A foundation establishment cultivator who learns and uses a rank 2 spell will not be able to channel the power of all of their life seeds into the spell.
