I'm playing a level 3 paladin and remember reading about spell or class feature that led me add my CHA mod in addition to my strength mod on melee damage rolls, but I cannot find it. Does anyone know what I'm looking for?
No, there is nowhere in the Paladin class feature which adds Charisma modifier to the damage roll.
There is actually a Channel Divinity feature called Sacred Weapon which raises your attack roll, for Oath of Devotion. By activating it, you are granted of attack roll bonus equal to your Charisma bonus (or +1, whichever is higher) for one minute. However, it only increases your attack roll, not your damage roll. If your DM was also applying it to your damage roll, your DM is either mistaken or houseruling.
In higher levels, Improved Divine Smite allows you to add 1d8 radiant damage to all your melee attacks, but that does not depend on Charisma modifier.