I think 3x3 is ok! Are you able to hand them out? Because they have to be at least 4x5 to mail I think.
It does kinda depend on the gift they gave too though...if it all added up to just a few $ a person, then definitely little thank you notes are fine...but if each gave like $20, I would maybe go for A2 size.
If it's to save time...maybe checkout CAS in the gallery...there are tons of VERY easy incredibly beautiful samples that take very little time.
HTH & Congratulations!!