I know magical and non-magical resistances do NOT stack, but to non magical resistances stack? Making an Aasimar Stormborn Sorcerer and was curious if at level 3 he would get resist 10 electric or if he would still only have resist 5?
Indeed. Resistance (and damage reduction, for we apply the same rules) aren't bonuses. Bonuses can stack, if they are different types, or untyped, etc.
Resistance scores and damage reduction don't have types, never stack, and it has nothing to do with the source