I think Mason, Brenna and Valerie sound fine together.
Well I don't want to be mean, harsh or rude, but I think Valerie would be really mismatched with Brenna and Mason. Valerie is a bit more dated, but I too like the name. Rather it as a middle though, so maybe you could consider that. But in the end it is your choice, who knows, maybe you don't even like matching names. I think of Mason & Brenna as siblings to:
Valerie sounds just fine to me with Mason and Brenna.
Happy to help, Kate! I don't think you should feel boxed in by Mason and Brenna, because I really do think so many names work, including Valerie - it could be deciding what level of "working together" you're willing to accept.
For what it's worth, I find Rachel classic, not really dated. If Natalie and Rachel are both or were on your list, you may consider: Abigail, Cara, Isabelle, Keira, Lily, Samantha, Scarlett...