You'll need 2 taxis. Also, they sometimes don't like going to T1 as the fare is too low (unless you take the pre-paid cabs from the desk inside).

Best to take the shuttle with that much time. You won't be able to access the dept hall anyway until 3 hours before the flight. I also suggest you do not leave the T3 arrivals hall if you have a lot of spare time as there is a nice cafe, etc. where you can wait.