Well, another autoflower is certainly NOT your "only option" .... how silly! Personally, I would pop a photo to add into your grow.... the Magnum Auto is going to have her own schedule and require whatever length of time she requires - which isn't bad, mind... but if you were to pop a photo now, it would veg and veg during the time it takes your other 2 autos to finish and you could be topping away at the photo and have a massive plant which would be ready to flip when the autos are finished and harvested...
In the end, either option is fine - it's just going to depend on what you decide you want to do! Good luck - and happy holidays!!