Just a water change, there hasn't been much going in there to need a gravel vac yet and all that fish poop and plant matter is helping the tank mature at the moment. I think 20% would be plenty but the others may correct me. Since it's a big tank and only 3 fish I don't think they would produce enough waste to need to change out more. Unless you have been feeding them a lot, which is very easy to do.
If you are worried about the gravel looking unsightly with leaves etc then stir it up a bit and then suck the main stuff off but don't let the vac suck the stones for a few more weeks I reckon.
As Ira says, an ammonia kit would be a good investment and will help you keep an eye on things. If you can afford the whole testing kit then Ammonia, Nitrite and Nitrate are useful as you can see which stage of the cycle it's at.