if a tournament has 10 rounds, and I can only play the first 3 (and in
this example let’s say I win 1.5 points), will my initial rating
become the average of these 3 opponents or will this not count as a
part of my full initial rating?
No and no.
The FIDE rating regulations say
7.14 A rating for a player new to the list shall be published only if it meets ...
It depends on the players and how black continues after whites moves.
For me, I find it easy to beat players using it for black.
That might speak more to their lack of experience than the opening. But then you dont find many high rated players using it so there is no conclusive proof.
You should understand what your rating really is: A rough approximation of your recent performance against other players in the same rating pool. What's good on one server might stink on another, and none of it matters.