Infant Clothing Sizes?

This is my first child, and I’m trying to organize the clothes we have for him according to size. Only I just realized that some of them are just marked “3″ or “6″ or “9″. If something is marked “3″ does that mean it’s 0-3 months, or 3-6? I know it probably depends on who made the clothes, but a lot of the ones that are marked with just one number are Carters brand. I also know from baby-sitting other kids that the month is just a guideline (for example, I sat for a little girl who was 15 months, but wore stuff in the 18 months size) and that the clothes are really based on weight, but… I would like to keep all the clothes together and don’t know if I should put things marked 3 with my other things that are marked 0-3, or if they should be with the rest of my 3-6 stuff? I also realize this may be a dumb question, but like I said before it’s my first! :-)