Exalted is a triggered ability that fires whenever a creature you control attacks alone. For each instance of Exalted you have (across any number of permanents), that lone attacker gets +1/+1 until end of turn. So three sources of Exalted means a single attacker gets +3/+3.
The key nuance: "attacks alone" means exactly one creature is declared as an attacker—it doesn't matter who it's attacking, and it doesn't have to be a creature with Exalted itself. If you send in two attackers, Exalted doesn't trigger at all. Also, Exalted bonuses stack from multiple sources but each only grants +1/+1; they're not optional and all trigger simultaneously.
In Commander, Exalted shines in voltron strategies—buffing a single evasive or commander threat for big, often commander-damage-relevant swings while keeping the rest of your board back as defense.