[name_u]Elliot[/name_u], [name_u]Owen[/name_u], and [name_f]Anna[/name_f] make a great sibset! All of the names sound current, but also have a lot of history. Other ideas:
[name_f]Iris[/name_f]: [name_f]Iris[/name_f] is a gentle, beautiful nature name like [name_f]Violet[/name_f], and fits with [name_u]Elliot[/name_u] and [name_u]Owen[/name_u] both in sound and style. [name_u]Elliot[/name_u], [name_u]Owen[/name_u], and [name_f]Iris[/name_f].
[name_f]Irene[/name_f]: This name is peaceful, evocative, and gorgeous. [name_u]Elliot[/name_u], [name_u]Owen[/name_u], and [name_f]Irene[/name_f] are stunning.
[name_f]Aurora[/name_f]: This one is a bit more modern than some of your favorites, but it is gorgeous and feminine, like [name_f]Anna[/name_f]. I also picks up the “o” sound, and has great consonant sounds. [name_u]Elliot[/name_u], [name_u]Owen[/name_u], and [name_f]Aurora[/name_f].
[name_f]Ada[/name_f]: [name_f]Ada[/name_f] has a lot of strength and charm, just like your boys’ names. [name_u]Elliot[/name_u], [name_u]Owen[/name_u], and [name_f]Ada[/name_f] are well matched!
[name_f]Audrey[/name_f]: This name is beautiful, elegant, and classic, and never goes out of style. [name_u]Elliot[/name_u], [name_u]Owen[/name_u], and [name_f]Audrey[/name_f] all have great vowel sounds and have the same style. For a very modern name that is similar, what about [name_u]Aubrey[/name_u]?
[name_u]Avery[/name_u]: This name is so wonderful, and is popular and recognizable. It contains sounds from both [name_f]Anna[/name_f] and [name_f]Violet[/name_f]. [name_u]Elliot[/name_u], [name_u]Owen[/name_u], and [name_u]Avery[/name_u].
[name_f]Adelaide[/name_f]: The sounds of this sweet, lovely name really tie well together with [name_u]Elliot[/name_u] and [name_u]Owen[/name_u]. [name_u]Elliot[/name_u], [name_u]Owen[/name_u], and [name_f]Adelaide[/name_f] sound at once vintage and current. A similar choice that I might like even better is [name_f]Adeline[/name_f], as it is more tailored, like [name_u]Elliot[/name_u] and [name_u]Owen[/name_u]. [name_u]Elliot[/name_u], [name_u]Owen[/name_u], and [name_f]Adeline[/name_f] sound just about perfect together.
[name_f]Alexandra[/name_f]: A current favorite, and for good reason: It is regal, stylish, and elegant. [name_u]Elliot[/name_u], [name_u]Owen[/name_u], and [name_f]Alexandria[/name_f] sound great together. For a shorter pick, perhaps [name_f]Alexa[/name_f]?
[name_f]Hope[/name_f] this was helpful, and congrats!