Your questions are a bit confused. I'll try to answer them as I best understand them.
Each of the Disney dining plans offers a specific number of entitlements per person, per night. Those entitlements can be used for any meal (or snack, as appropriate) at a participating location. It does not mean that you simply eat anything and everything you want for free -- you have a fixed set of allowances based on your entitlements.
There are three different dining plans, each offering a different set of entitlements. The only one that offers three meals per guest per night is the Deluxe Dining Plan (DxDP), the others offer two meals per guest per night.
The meal and snack credits are put onto your room keys, so you can "pay" with them. All credits for the room are accessible from any key. The credits are awarded in a lump sum when you check in, so you can use them any time, and in any order, you like until midnight following your check out day.
For full details, check out the link that Nala provided above.