When your car or vehicles engine is leaking oil or burning off some oil every few hundred miles or so and you top the oil off then you won't usually need to change the engine oil every 3,000 miles or as often as you would otherwise because you're replenishing the engine with some fresh oil.
But still I would change the engine oil every 6,000 miles at most even when you're topping off the engine oil.
You still have old contaminated engine oil in your vehicles engine oil pan that needs changed and your oil filter should be changed as well during the oil change.
If you don't change your engine oil until 6,000 miles then you should at least still change the oil filter because after 3,000 miles the oil filter is ready to be changed and could be partially clogged.
The oil filter and oil is cheap to change out compared to the cost of a breakdown and tow bill as well as the cost of a replacement engine or replacement car or other vehicle.
I still change my oil and oil filter every 3,000 miles because I want to make sure my engine and vehicles lasts a long time.