Hello, I hope this helps you, and I hope you are able to find the name that feels right for your 3rd boy! Give yourself grace, naming a baby (especially when you have to compromise) is way easier said than done!
I love your naming style! I get romantic, vintage vibes from Edmund and Ambrose, so my suggestions are along those lines:
Romeo (Italian; Roman - nn Rome, Romy 
)
Marshall (French; Caretaker of Horses - nn Marsh, Mars 
)
Clarence (Latin; Bright and Clear - nn Ren 
)
Barnaby / Barnabas (Greek; Son of Consolation - nn Barry 
)
Oscar (Irish; friend of Deer - nn Oz 
)
Wilbur (German; Resolute and Briliant - nn Will 
)