Nostalrius Begins - Quality wow vanilla realm (1.12)

So in the Discipline Talent tree of priests, you can unlock Divine Spirit(rank1)

At the class trainers around level 40, you can unlock Divine Spirit(rank2) and later Divine Spirit(rank3).

Rank 2 requires you have Rank 1. And Rank 3 requires you have Rank 2.

-----------------------------------------------

Now what I want to know is, Do I need Rank 1 from the talent tree to be able to use Rank 3?
I earn Rank 1 from the talents. I go buy rank 2 and then 3.
Then I respec and go all into the Shadow tree. I won't have Rank 1 anymore.
Will Rank 2 and 3 still function after I bought them?

I'm new obviously, trying to understand how my talent choice will affect me.

I'm afraid not, if you unlearn the talent the spells will dissapear for that talent and when you relearn it they will reappear.