The first combo (1.Unchained - 2.Seething Howl - 3.Brutalize) does allow for huge amounts of burst damage. You chase down and enemy(or join a team fight) with Unchained, buff yourself with Seething Howl and, hopefully, land 4 big hits on an enemy. That can easily kill characters with low health pools or low health due to damage they have already taken. A downside is that you don't have a way of escaping until you can use Unchained again.
The second combo I don't particularly like or get why you would do it. 2.Seething Howl is a damage buff, so if you are using Unchained as a means of escape you are rendering your buff useless. A possible use for that combo is if someone manages to escape your Brutalize. Then you can easily finish them off with a buffed Unchained.
I prefer initiating with a Brutalize on flanked enemies but that depends on your positioning and the ability to get to lone players. Fenrir is a single-target damage hero, so you should focus on doing big bursts of damage and leaving the fight as fast as possible. 2-3-1 is also a valid option since it starts with a buffed, high-damage ability and leaves Unchained for chasing or escape.