I'm not expert on steelhead, but as I understand it's a combination of trout and salmon. With either fish, a brine would do you good, but personally think 3 days is a little steap. For fillets, I would think a few hours would be sufficient. Make sure to rinse well and re-season.
As for cooking, for handling purposes, I would cook them on a wet cedar plank. Cook until it flakes with a fork.
At most brine them for 24 hours which is on the steep side. Take them out, rinse, pat dry and put them on a rack so that air can circulate on both sides. Leave in the fridge until the pellatin has formed.