Each of the above is true, bringing your death toll to 5. However, there are some additional details worth noting.
Mercutio, the first death, occurs because Tybalt wants to fight Romeo and Romeo won't fight. He starts talking about how he loves Tybalt and Tybalt has no comprehension of why Romeo is talking like this, which makes sense because Romeo and Juliet are keeping their marriage a secret. Mercutio can't stand to see Romeo be such a push-over and tries to instigate a fight with Tybalt. Romeo tries to break it up and Mercutio is hurt "under [Romeo's] arm."
Romeo kills himself because he thinks Juliet is dead. She really is getting ready to wake up.
Juliet kills herself because she knows Romeo is dead.
When Lady Montague hears of all of this, she cannot bear to take any more of the feud between the families and her only son's death scarred her more than she imagined. Thus, she seems to have chosen to take her own life.
Mercutio dies of stab wounds from Tybalt.
Tybalt is slain by Romeo.
Romeo kills Paris at the Capulet gravesite.
Romeo drinks poison and dies.
Juliet stabs herself.
At some point, Lady Montague died. Her death is reported in Act 5.
If we are speaking of deaths caused by events occurring in the play, then possibly the Apothecary who sells Romeo poison, as selling poison is punishable by death in Mantua.
146,746,141,995 people died.