Because Lyon was a question mark, and Husso with his injuries was also a question mark. Therefore, get a 3rd one as insurance.
When you listen to Derek and Steve talk about running 3 goalies, IMO it sounds like they like this system. They certainly fell into it last year, but it seems like they were pleased with it and are happy to keep it going.
And TBH it's kinda smart. Teams don't really have roster space to claim goalies. So Steve can waive and recall at will, until another team is in need of an injury replacement.
I see Steve doing this system until basically our Vasilevsky emerges.
If anything we should get a FOURTH goalie.