I would be asking about taking DS2 to wedding as he is only little. I was worried about people feeling they couldnt bring babies to ours and made it clear to the 3 with babies that they were very welcome! My MOH had a 9 month old, and another friend at 8 month old and then a 6 week old.
Most people are generally ok with a baby its more the annoying toddler age to young child they don't want at a wedding.

Otherwise thought I think 1 nanny would be fine with 3 kids. But are they going to be in a hotel room? appartment??