I like the names you’ve listed, particularly [name]Milo[/name].
Here are some other suggestions to go with [name]Sebastian[/name], [name]Flynn[/name] & [name]Oliver[/name]:
Of the three you mention, [name]Milo[/name] works by far the best with the other brothers’ names. I also think it sounds best with [name]Christopher[/name].
Besides [name]Milo[/name], others that I think work are:
[name]Sebastian[/name] [name]John[/name], [name]Flynn[/name] [name]Alexander[/name], [name]Oliver[/name] [name]David[/name], and [name]Gabriel[/name] [name]Christopher[/name]
[name]Sebastian[/name] [name]John[/name], [name]Flynn[/name] [name]Alexander[/name], [name]Oliver[/name] [name]David[/name], and [name]Noel[/name] [name]Christopher[/name]
[name]Sebastian[/name] [name]John[/name], [name]Flynn[/name] [name]Alexander[/name], [name]Oliver[/name] [name]David[/name], and [name]Julian[/name] [name]Christopher[/name]
[name]Sebastian[/name] [name]John[/name], [name]Flynn[/name] [name]Alexander[/name], [name]Oliver[/name] [name]David[/name], and [name]Hugo[/name] [name]Christopher[/name]
[name]Sebastian[/name], [name]Flynn[/name] and [name]Oliver[/name] are so perfect together! Of the names you have listed I think [name]Milo[/name] slots in perfectly, it’s not overly popular but it’s widely recognised. Would you consider making [name]Miles[/name] nn [name]Milo[/name]? That would be perfection [name]IMO[/name].
[name]Harry[/name] fits with [name]Oliver[/name] because both are so, so popular at the moment but I dislike it a lot.