Initial FIDE rating Question According to FIDE, if I win half of my games in my first FIDE tournament, then my rating = the average of my opponents ratings.
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?
Looking through the handbook, I see no rule that says I can’t just leave a tournament after drawing the first round against a 2000 and my rating becoming 2000 (because the average of 1x2000 player/#of players = 2000.
Can anyone clarify from the rules exactly what determines an initial rating given one does not fully finish a tournament?
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 the following criteria:
7.14a If based on results obtained under 6.3, a minimum of 5 games.
7.14b If based on results obtained under 6.4, a minimum of 5 games played against rated opponents.
7.14c The condition of a minimum of 5 games need not be met in one tournament. Results from other tournaments played within consecutive
rating periods of not more than 26 months are pooled to obtain the
initial rating.
7.14d The rating is at least 1000.
7.14e The rating is calculated using all his results as if they were played in one tournament (it is not published until he has played at
least 5 games) by using all the rating data available.

So, if you only play 3 games you will not have a rating. However if you later play another tournament then the results of those 3 games will be carried forward.