Yes, it is normal for your phone or the charger to get warm whilst charging however should not get to the point where either are too hot to touch.
This may be caused by one of the below reasons:
Your phone is not being placed in the exact position that it needs to be, this is important as the phone needs to be aligned with the charging coil in the charger to charge efficiently
You are not using a SnapMag ring on the back of your phone or case (if it is placed on the back of your phone, you will not be able to use a case. If you place it on the back of your case you can not place anything in between your phone and the case) The SnapMag ring needs to be in direct contact with the charger.
You are using your own case and it is too thick meaning there is little to no energy transfer
You are using the AirVent attachment for your SnapMount 3 and have your heater on high