are any of these appropriate for a formal 15th birthday party?

I would choose the third one definately,

then the second

and lastly the first.

Love the colour of the third one and it's very formal. White flats or heels would go perfectly with the dress. Gold is a possibility, but it might be too much metallic. Silver, maybe but also too much metallic.

I would pair the second dress with cute soft pink flats/heels or white ones.

First dress, I would wear it with white/navy/yellow flats,heels or pumps.

Hope I was helpful ~

the last one

the last one is the most formal. i like the first one. the first one go with red shoes. the second black shoes. the last one either black or whatever your necklace color is.

have fun.

i like the first one. you should wear red or bright yellow shoes :]