Minimum 3 to ship means you can't buy less than three of that kind ("kind" being my word for type within one breed) and have them shipped to you, even if you are ordering other chicks at the same time. It doesn't matter how many other chicks you order at the same time.
So, I could not buy one buckeye pullet or one heritage rhode island red pullet or one black ameraucana pullet even if I bought other chicks in the same order.
You can order 3 buckeye straight run as your total order. You would pay a small order surcharge of $20 in addition to the cost of the chicks and the shipping charge.
Or you can order 15 chicks or more and not pay the small order charge, they just have to include at least three of any given type of the ones with that designation.
For the most part, pullets and straight have that designation and males do not. I think there are some exceptions to that but I looked at just a couple now to see if it was generally the same; I haven't looked at nearly all of the breeds since last year, I think.
You aren't picking them but to be more complete for anyone else, I think they worded it that way because you can get 1 of a given kind if you pick them up at Cackle. I'm not positive because I wanted at least 3 anyway when I picked up mine.