In the first 3 hours the pool gets 50 gallons/hour x 3 hours = <<50*3=150>>150 gallons
With the addition of the other hose, the pool was filling with a rate of 50 gallons/hour + 70 gallons/hour = <<50+70=120>>120 gallons/hour
In the next 2 hours with both hoses the pool gets 120 gallons/hour x 2 hours = <<120*2=240>>240 gallons
The pool's total capacity is 240 gallons + 150 gallons = <<240+150=390>>390 gallons
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