Only Barcelona have a perfect points so far (9 points from 3 matches), no one would be wrong to expect them to become La Liga champions this season, but that's too soon to say, we haven't even go to the first El Clasico of the season yet, so even though Barcelona are currently having a good run, we can't know how they will be after that.
About Barcelona's match against Rayo Vallecano, I thought Olmo played well, it's a shame he didn't play from the start, maybe if he played earlier we could have seen Barcelona's third and fourth goals.
As long as Barcelona are leading by less than three points and until they meet Real Madrid head-to-head, bookmakers will continue to rate them as clear underdogs against Real Madrid (which are still priced at ~1.50 even after dropping points). The fact that they have 9 points is not bad, but you can’t say that they are winning confidently - three minimal victories and in every game they are conceding goals.
The season is still quite long and at this time I think it has not proven anything so Real Madrid is more favored because of the overall strength of both teams and I think it is quite worthy.
Barcelona's winning streak in the first 3 matches is indeed quite impressive, and they show impressive performance and quality players in all lines but I think there is no guarantee that they will be able to continue to be consistent with the game especially Barcelona has not faced the real test against the top La Liga teams so I still favor Real Madrid overall in terms of the title race this season.
And for Barcelona's new players, what Olmo showed in his debut match was quite impressive, this could be an attraction for Olmo to be better in the future and contribute greatly to the Barcelona team.
The winning goal scored by Olmo will have a big impact on the player's confidence.