How does Triple Stab calculate?

Daggers and Blades have different formulas, formula shown is the both combined in terms of weapon attack.
Triple Stab does one Dagger, one Blade, and one more dagger hit

Minimum Damage = [(3.6 * 0.9 * Mastery * LUK + STR + DEX) / 100] * WA
Maximum Damage = [(3.6 * LUK + STR + DEX) / 100] * WA

Apply both to Dagger/Blade to get your ranges.