We would like to ask how far the distance from terminal 3 to terminal T2 in Capital Airport.

It is about 4.5 miles between these terminals, and there are free shuttle buses available, and the bus ride is about 10 mins.
You need at least 120 mins for the transfer, and you have enough time, do not worry about it. The transfer procedures is like this: Inspection & Quarantine, Exit and Entry Frontier Inspection -- Baggage Claim -- Customs Inspection -- Going through Transfer Formalities -- Security check -- Waiting & Boarding