Luis Suarez, Arturo Vidal and Dembele were all on target in eight clinical first-half minutes as Barca made Sevilla pay for missing a string of early chances. Lionel Messi, with what was his first goal of a stop-start season, then rounded off the scoring with a free kick in the 78th minute before referee Mateu Lahoz took centre stage.
Lahoz sent off Araujo in the last minute for a last-man challenge on Javier Hernandez and then dismissed Dembele for something the Frenchman said as he protested the decision.
There were big question marks over Barca when they fell to Granada two weeks ago but they've responded with four wins on the spin. The last two, against Inter Milan and now Sevilla, have been impressive for different reasons. They go into the international break two points behind leaders Real Madrid.
The win over Sevilla was a lesson in efficiency. Suarez may have his doubters but he demonstrated he's still a master in front of goal with a brilliant bicycle kick to open the scoring. Vidal, Dembele and Messi also took their chances well.
Barca have struggled to score the opening goal this season and only poor finishing from Luuk de Jong prevented Sevilla from taking the lead. The Dutch striker missed three great chances before Suarez's opener and hit the post with another effort after the break. Barca remain fallible in defence.
Two late expulsions also left a sour taste. Araujo won't be missed while suspended, but it's a shame for him that his debut ended with a red card, especially when the contact with Hernandez was minimal. Barca will also be sweating on how long Dembele will be banned for -- a lot will depend on what Lahoz includes in his report.
Manager rating out of 10
6 -- Ernesto Valverde made a couple of big calls, notably leaving out Sergio Busquets and Antoine Griezmann. They look like good decisions looking at the result, but it could have been different if Sevilla had taken their chances.
Player ratings (1-10; 10 = best. Players introduced after 70 minutes get no rating)
GK Marc-Andre ter Stegen, 7 -- A stunning save from De Jong early on provided the platform for Barca to run riot in eight first-half minutes.
DF Jean Clair Todibo, 6 -- With Clement Lenglet suspended and Samuel Umtiti injured, the French teenager was thrown in for his first start of the season. Struggled positionally at times but was comfortable and confident in possession.
DF Gerard Pique, 6 -- Played as the left-sided centre-back when he's usually on the right. Did OK and helped Todibo through a potentially tricky game.
DF Nelson Semedo, 8 -- A much-improved performance at left-back from the right-back. Lovely cross for Suarez's opener and fancy footwork in the build-up to the second goal, too.
MF Frenkie de Jong, 7 -- Looking at home in Barca's midfield. One fabulous turn in the move that led to the second goal was complemented by assured passing throughout the night.
MF Arturo Vidal, 6 -- Earned his place with a brilliant cameo against Inter and delivered once again. Adds an attacking threat from midfield and timed his run perfectly to score Barca's second.
MF Arthur, 8 -- Another good performance from an increasingly undroppable player. Strong on the ball and clever with his passing, setting up two of Barca's goals. Almost undid some of his good work by gifting De Jong a chance for Sevilla, but the striker could only find the post.
FW Lionel Messi, 7 -- Deserved his goal after an injury-hit start to the campaign. Still not 100 percent match fit, but still a menace for opposition defences. Almost scored a wonder goal before finally netting via a free kick.
FW Luis Suarez, 6 -- Once again, his goals did the talking. His best years look in the past in general but his finishing has not diminished.
FW Ousmane Dembele, 5 -- Scored a brilliant goal and produced moments of skill that brought Camp Nou to its feet. But he needs to stop giving the ball away so much and Valverde will be angered with the way he earned his red card.
MF Sergio Busquets, 6 -- Came on with Barca 3-0 up to help close out the three points.
MF Ivan Rakitic, 6 -- Given a strong reception by the crowd and almost scored, chipping onto the top of the net.
DF Ronald Araujo, N/R -- A nightmare debut for the 20-year-old Uruguayan. Sent off for a foul on Hernandez.