score:3
For a single ticket, 90 minutes is plenty, if there are no major delays. You will be coming in either in Terminal 2 or 3 and departing from Terminal 3. Even if you come in at Terminal 2 you can take the Skytrain, which is quick and easy.
See https://www.changiairport.com/en/airport-guide/transport/transfer-between-terminals-and-jewel.html