Philo : I like the familiar, but different sound of this one. I've actually never heard it before.
Oswin : This sounds much better with Philo and Asa instead of Oswald. Besides, Oswald makes me think of the blue Octopus.
Asa : Not my favorite, but it's soft and short, balancing out the other two names.
I think they work well as a sibset. I would introduce them like "These are my darlings Philo, Asa, and Oswin."
Not crazy about these middles, but I would put them together like so:
Just gonna be upfront: Philo sounds like a dogs name and on Oswald I have to agree with pp about Lee Harvey.
Asa is a very nice strong and gentlemanly name. It was my great grandfather's name. It's also religious if that's a plus for you.
Not a fan of Prosper or Solace. But Noble's alright. Asa Noble is cute :)
I know Philo isn't everyone's cup of tea, but it has a bit of sentimental value to me. (For the record, the pastry dough called Phyllo is pronounced fee-low).
I like Oswin, but it almost feels too fangirly, as I'm a doctor who fan. I've always loved the name Oswald...so much that i actually had a cat i named that as a kid. I'm not really concerned about the Lee Harvey Oswald connection, and i don't have TV so my future kids won't even know about the blue octopus connection.
The middles are just ones i like. My girls list has middles that honor family, and i wanted boy middles that did the same without having to use some rather unattractive family names. I also like Makepeace and Hopestill for middles.