Ooh I think you have heaps of options here! Especially if you want them to be a little bit matchy…
[name_u]Rowan[/name_u] is an awesome celtic name so you could keep to that and have:
[name_u]Rowan[/name_u], [name_f]Saoirse[/name_f] and [name_u]Rhys[/name_u]
[name_u]Rowan[/name_u], [name_u]Camden[/name_u] and [name_m]Knox[/name_m] (these are all also nature names)
[name_u]Rowan[/name_u], [name_u]Riley[/name_u] and [name_u]Teagan[/name_u]
You could go all R-sounds + unisex:
[name_u]Rowan[/name_u], [name_u]River[/name_u] and [name_m]Rune[/name_m]
[name_u]Rowan[/name_u], [name_u]Arden[/name_u] and [name_u]Wren[/name_u]
Soft, hippie sounding names would be cute:
[name_u]Rowan[/name_u], [name_u]Shiloh[/name_u] and [name_m]Bodhi[/name_m]
Tree name theme:
[name_u]Rowan[/name_u], [name_f]Hazel[/name_f] and [name_u]Ash[/name_u]
[name_u]Rowan[/name_u], [name_u]Juniper[/name_u] and [name_m]Silas[/name_m]
[name_u]Rowan[/name_u], [name_f]Clementine[/name_f] and [name_u]Quince[/name_u]
These all have cute meanings as a group:
[name_u]Rowan[/name_u], [name_u]Piper[/name_u] and [name_u]Teagan[/name_u] (little red head, little flute player, little poet)
[name_u]Rowan[/name_u], [name_f]Scarlett[/name_f] and Jarrah (all have the colour red as part of their meaning, Jarrah is a type of red tree in australia like [name_u]Rowan[/name_u]!)
Few other ideas:
[name_u]Rowan[/name_u], [name_f]Freya[/name_f] and [name_u]Shiloh[/name_u] (all soft sounding but unusual and easy to say)
[name_u]Rowan[/name_u], [name_u]Phoenix[/name_u] and [name_m]Bodhi[/name_m] ([name_u]Phoenix[/name_u] and [name_m]Bodhi[/name_m] both have an awakening as part of their meanings so match in a subtle way)
[name_u]Rowan[/name_u], [name_f]Lila[/name_f] and [name_m]Knox[/name_m] ([name_f]Lila[/name_f] means night and [name_m]Knox[/name_m] is a spell to turn off lights in harry potter lmao)
[name_u]Rowan[/name_u], [name_u]Lennox[/name_u] and [name_m]Knox[/name_m] (latter two have similar nature meanings and share an X sound)
I could keep going but lol this will do haha