Barcelona are 8th position on the table, but you should know it's only 5 points behind 1st placed Atletico and 3 points behind 2nd place Athletic Bilbao, all these could be easily covered in 2 and 1 game(s) respectively, so for now, nothing has actually been guaranteed.
Yes, unless in 1 or 2 games they keep losing and the others keep on winning. See what happens to Liverpool last year. Even if the differences in just one match, it decided how they end the season.
Five points must be considered a massive gap if anybody wants to win the title. Feeling confident because the difference is only '5' will cost them if they're not careful.
5 points will never be a massive gap in any league after just 3 games played If it's an underdog topping the table. I'm sure Barcelona and Barcelona will be aware of the points gap, and won't take it for granted all I was saying is that it's way too early for the title to be decided just based on those points gap.
And you should also know Liverpool led with about 7 points if not more after 18 games has been played, yet still didn't win the league.