In extreme heat, you need electrolytes in addition to drinking more. Plain water doesn't stay with you and can't do its job. Prehydrate with an electrolyte enhanced drink, make one of your bottles an energy drink (even Gatorade has electrolytes). If you need more, take some type of salt tablets (like Enduralytes).

Many people don't know this, but you can actually overhydrate and get water poisoning if you aren't getting salts, potassium, calories, etc. Not a dangerous factor in a 2 or 3 hour ride, but people have died from it during multi-day endurance events.