We're Smiths too.

DH has three names, I have three names, DS has three names.

I have two middle names picked out for a second son and the daughter's name we like is three names.

Personally, I'd go with the three names.

I have not had any problems with forms (well aside from the fact two of my names have "funny" spellings even though I don't find them that amusing) for any of us. Only "problem" was DH's old bank would only use two initials on his cards. Our current bank uses all our name on the cards. The names aren't too long so will fit in just fine.