Title is self explanatory, need name for 3rd child, older siblings are [name_m]Marshall[/name_m] “Mars” [name_u]Vaughn[/name_u] and [name_u]Sierra[/name_u] “SJ” [name_f]Josephine[/name_f]. Last name is 2 syllables, stars with an A, prominent D sounds.
Other boy names I like-
[name_u]Spencer[/name_u]
[name_m]Ambrose[/name_m]
[name_u]Jensen[/name_u]
[name_u]Rhys[/name_u]
[name_u]Callum[/name_u]
[name_m]Lachlan[/name_m]
[name_u]Beckett[/name_u]
[name_m]Bruce[/name_m]
[name_u]Casey[/name_u]
[name_m]Conrad[/name_m]
[name_u]Edward[/name_u]
[name_u]Elliot[/name_u]
[name_u]Elmer[/name_u]
[name_u]Finn[/name_u]
[name_u]Flynn[/name_u]
[name_m]Hugh[/name_m]
[name_m]Kellan[/name_m]
[name_m]Norman[/name_m]
[name_u]Owen[/name_u]
[name_u]Walter[/name_u]
[name_u]William[/name_u]
[name_m]Zayne[/name_m]
[name_m]Marshall[/name_m] and [name_u]Sierra[/name_u] 
I like:
[name_m]Ambrose[/name_m] ‘Bram’?
[name_m]Lachlan[/name_m]
[name_u]Callum[/name_u]
[name_u]Jensen[/name_u]
I’ll suggest:
[name_u]Callahan[/name_u] ‘Cal’
[name_m]Warren[/name_m] ‘Wren’ / ‘Ren’
[name_u]Franklin[/name_u] ‘Finn’ ‘Frankie’ ‘Flynn’
[name_u]Declan[/name_u] ‘Dex’ ‘Dax’
[name_m]Jarrett[/name_m] ‘Jett’
[name_m]Patrick[/name_m] ‘Pax’
[name_m]Caspian[/name_m] ‘Cas’ ‘Cap’ ‘Kip’
[name_u]Leland[/name_u] ‘Landry’ ‘Len’
[name_u]Valentine[/name_u] ‘Van’ ‘Vinnie’ ‘Len’
[name_u]Barrett[/name_u] ‘Bear’
[name_u]August[/name_u] ‘Gus’
[name_m]Kurtis[/name_m] / [name_u]Curtis[/name_u] ‘Kit’
[name_u]Beckett[/name_u] ‘Beck’ ‘Kitt’
[name_u]Robin[/name_u] ‘Bobbie’
[name_m]Marshall[/name_m], [name_u]Sierra[/name_u], and…
[name_u]Parker[/name_u]
[name_u]Wesley[/name_u]
[name_u]Reid[/name_u]
[name_m]Vincent[/name_m]
[name_u]Kieran[/name_u]
[name_u]Dorian[/name_u]
[name_m]Cyrus[/name_m]
[name_u]Rhett[/name_u]
[name_u]Declan[/name_u]
couldn’t resist, as another [name_m]Marshall[/name_m] “Mars” 
I think this is the first time I’ve seen someone else sharing my name/nickname combo, and coincidentally a middle initial. pretty cool!
Of your list, I think [name_u]Jensen[/name_u], [name_u]Callum[/name_u], and [name_m]Lachlan[/name_m] fit well in the set! I could see [name_u]Rhys[/name_u] working as well.
some ideas…
[name_u]Rhett[/name_u]
[name_u]Connor[/name_u]
[name_u]Logan[/name_u]
[name_u]Arthur[/name_u]
[name_m]Trenton[/name_m]
[name_m]Aaron[/name_m] (maybe too similar in sound to Sierra)
[name_m]Laszlo[/name_m]
[name_m]Kellan[/name_m]
[name_m]Patrick[/name_m]
[name_u]Rhodes[/name_u]
[name_u]Jasper[/name_u]
[name_m]Graham[/name_m]
[name_u]Emmett[/name_u]
[name_u]Reed[/name_u] / [name_u]Reid[/name_u]
[name_u]Nolan[/name_u]
[name_m]Isaac[/name_m]
[name_m]Stellan[/name_m]
[name_u]August[/name_u]
[name_u]Jude[/name_u]
[name_u]Quentin[/name_u]
[name_m]Tobias[/name_m]
[name_m]Heath[/name_m]
[name_m]Newton[/name_m]
Congrats on #3!
Of your names, I think [name_u]Callum[/name_u], [name_m]Ambrose[/name_m] or [name_m]Lachlan[/name_m] round out the sibset the best!
Try:
[name_m]Marshall[/name_m], [name_u]Sierra[/name_u], & [name_m]Dominic[/name_m] “Mars, SJ, & Nicky/ [name_u]Nic[/name_u] or even Domino” 
[name_m]Marshall[/name_m], [name_u]Sierra[/name_u], & [name_m]Lucien[/name_m] “Mars, SJ, & Lars/Lucky/Luca”
[name_m]Marshall[/name_m], [name_u]Sierra[/name_u], & [name_u]Callan[/name_u] “Mars, SJ, & Cal”
[name_m]Marshall[/name_m], [name_u]Sierra[/name_u], & [name_m]Kendrick[/name_m] “Mars, SJ, & Rick/Ricky/Kenny”
[name_m]Marshall[/name_m], [name_u]Sierra[/name_u], & [name_m]Anthony[/name_m] “Mars, SJ, & Tony/Andy”
Hopefully these help 
I like [name_u]Spencer[/name_u] (Spence) & [name_m]Lachlan[/name_m] (Lock) as potential brother names.
Ideas:
Archer/Archibald or [name_m]Barclay[/name_m] (nn Ark/Arc)
[name_u]Hendrix[/name_u] (Rex)
I like [name_u]Spencer[/name_u], [name_m]Ambrose[/name_m] and [name_u]Jensen[/name_u]!