Three AAAs will be fine. Just remove the battery clip and there is plenty of room for the holder. They use 9v connectors. You can get a pack of 5 from Radio Shack for about $2.50. Sometimes a 4.5v shutter will work with a 3v battery but don't count on it. The Polaroid repair manual calls for a minimum of 4.1v and I've seen shutters that won't work with 4.0v. Quality has been spotty on the 4.5v cells, especially those made in China. I bought a batch and most were less than 4v. The vendor replaced them but I no longer trust the quality.
For 3v cameras I prefer CR123s. They last longer and never leak. Good batteries should last for years in a Polaroid.