The third is at Deas Dock right now. I believe it’s just intended to supplement the Tsawwassen-Southern Gulf Islands route.
The Salish-class at Deas is there for its warranty repairs, upgrades and a few changes.
There will be an "extra one" off season. However with relief for refits and returning to service for off-season peak periods I doubt we will see it spending too much time at dock which is good. The Bowen Queen, however, will now barely sail though (which is a good thing that its service is minimized but BC Ferries did not just retire it - they've been running way too thin for the last decade).
On Sundays in the summer the Salish Eagle will come to Swartz Bay assuming the schedule doesn't change; I don't think it will for 2018.