Firstly love [name_f]Fern[/name_f] 
I have [name_f]Fern[/name_f] [name_f]Effie[/name_f] Lockhart on my list both middle names are family names for me but I love this little combination. I think I would probably choose something longer in the middle spot to balance the names out.
[name_f][/name_f]Suggestions
[name_f][/name_f][name_f]Fern[/name_f] [name_f]Seraphina[/name_f]
[name_f][/name_f][name_f]Fern[/name_f] Dorothy/Dorothea
[name_f][/name_f][name_f]Fern[/name_f] [name_f]Romilly[/name_f]
[name_f][/name_f][name_f]Fern[/name_f] [name_f]Clarissa[/name_f]
[name_f][/name_f][name_f]Fern[/name_f] [name_f]Augusta[/name_f]
[name_f][/name_f][name_f]Fern[/name_f] [name_f]Ophelia[/name_f]
[name_f][/name_f][name_f]Fern[/name_f] [name_f]Leonora[/name_f]
[name_f][/name_f][name_f]Fern[/name_f] [name_f]Charlotte[/name_f]
[name_f][/name_f][name_f]Fern[/name_f] [name_f]Aurelia[/name_f]
[name_f][/name_f][name_f]Fern[/name_f] [name_f]Matilda[/name_f]
[name_f][/name_f][name_f]Fern[/name_f] [name_f]Elisabeth[/name_f]
[name_f]Hope[/name_f] this Helps