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.
Yes it is true that they are the only team that has been most successful until week 3, they can get perfect points by winning all the matches they play. But keep in mind that this is only week 3 and there are still many matches they have to play, we don't know if they will be consistent enough to keep winning every match or they have to decline because they have many problems, injury storms for example.
You are right, they will also still have 2 away home matches against Real Madrid, even they will still face Atletico Madrid, that might give them a defeat. Apart from that, there might be other teams that are surprising this season like Girona did last season.