Yes, You can use the charger, they are the same
But when charging my 3t phone it get heat( a little bit warm) will the charger is good or bad for it?
As @Cervantes mentioned, the OnePlus 6 charger can be safely used with any OnePlus device. With the OnePlus 3T, it will work as a regular Dash Charger.
OnePlus Plus Plus: Redesign a OnePlus Product
by Trista W., 2019-12-20