The difference is just in the size of the pieces. Pellets looke like tiny little tubes or rectangles. Crumbles look like tiny pieces of granola. Mash is either crumbles that are too small to be called crumbles, or pellets/crumble/mash with water added to it (depending on where you live).
I don't know anyone who feeds mash. Most people feed crumbles to small or young birds, and switch to pellets when they are larger.
i do pellets for my hens and my roosters who i have seperated out that are going to freezer camp will be starting in a couple weeks on higher protien turkey feed to help beef them up* tried chick grower they arent real fond on it anymore* lol
The only difference is the size of the bits. They grind all the stuff that goes into the feed together and mix it. That's called mash. To make pellets they get the mash wet with water, push it through a mold, and dry it. To make crumbles, they crush the pellets to get them to crumbles size. If you crush them too much, you would be back to mash.
Baby chicks need either mash or crumbles. They can't handle the pellets, though a lot of people would be amazed at how fast they can handle pellets.
The main reason for the different forms of the same feed is for the delivery system. If you use automatic feeders, some feeders work better with mash, some with crumbles, and some with pellets.