Please critique my arena team!

Soren seems like a great choice for DD3. I think all three of these proposed changes would improve the team, except that if you're going to leave Fury3 on Marth and take Renewal 3 off, you might also want to change reciprocal aid for something else. Swap or a double rally maybe? Removing 10 points of healing on a unit with Fury will result in a dramatic loss in healing ability.

I run my 40+5 masked Marth with desperation in the B slot, and heavy blade in the S slot to make up for the lack of getting hit. But I see that team and it makes me feel like I need to up my game.

If you have the spare divine dew, I would also consider giving Masked Marth Sealed Falchion instead. The Awakening Falchion refine isn't the best on Masked Marth and Lucina since their stats set them up to be offensive and bond skills relegate them to a defensive enemy phase role. But if you give Masked Marth Sealed Falchion, switch out renewal for desperation and reciprocal aid for reposition or whatever your team needs. For the seal, heavy blade should work fine, but you could always switch it out for attack or speed +3.
For Soren, I would probably switch out vantage for green tomebreaker. He's not the best at using vantage and tomebreaker lets you check the countless Ninos in this tier. I would also switch out swap for draw back (I also just really don't like swap personally). Distant Defense seal would also help a lot in tanking Reinhardts and Ninos.
Your Nowi should be working perfectly fine the way she is, so there really isn't much I would change.

Remember though, these are only my suggestions! I'm not you and I don't know how this team specifically works. You know you're own team the best, so take all my critiques with a grain of salt and do what you think would work the best for you.