Advantage is additive while Disadvantage is subtractive. In the situation where a character has two sources of Advantage and one Disadvantage, the resulting roll will be made with advantage..

However, the same type of Advantage from 2 distinct sources (I.E two different characters using the help action for example.) do not stack and only count as one for the purposes of calculating if you have advantage or not.

When you have taken up  position on a house, or in a tree, you can gain bonuses:

 

Prone Negation: When you perform a ranged attack a creature that is prone that is within a horizontal distance that is less than or equal to your elevation, you negate the disadvantage from attacking the creature.

Attacks Affected: When you attack an enemy from an elevated position, you gain +2 bonus to the attack roll.