There is no train yet. I would use Grab. You can pre-install the app on your phone and just buy a local SIM card at the airport just outside the arrival gate but still inside the building.
If waiting is not a problem for you, there was a Grab booth in Terminal 3 last May 2019 when I last visited. (Grab Booths).
When I was there the line was very long, and I didn't want to wait so I walked across to the other side and booked my ride from there. There is an elevator that you can use to access a footbridge to the other side, and then book your Grab ride from outside McDonald's.