Dot 3 &4 are Glycol based & absorb water. Dot 5 is silicone based and does not absorb water. You state that you are topping off. Do not use Dot 5 to top off brake system with Dot 3 already installed. Actually consider draining & flushing the Brake system & refill with Dot 5.
Dot 3 & 4 also have lower boiling points then Dot 5

Bob