From PackRat Recipe Wiki

A recipe is three cards combined together to make a new, higher value card. When you have three cards that make a recipe the "Make a Card" option will be lit up at your pack. Recipes are made from cards that are (usually) all from the same collection. The Dreams and Nightmares collections use cards from each other to make recipes.

There are seven different kinds of recipes.

A + B + C are three different cards from the same collection combined together. Example:

A + A + B are two identical cards from the same collection combined with a third different card. Example:

A + A + A are three identical cards from the same collection combined together. Example:

A + B + D are two cards from one collection and one different card from a different collection combined together. Example:

A + D + E are one card from one collection and two different cards from a different collection combined together. Example:

D + E + F are three different cards from a different collection combined together. Example:

D + D + E are two identical cards from a different collection combined with a third different card from that collection. Example: