What does pattern stitch multiple of 3 stitches mean?

That means the number of stitches in the row repeat pattern is a multiple of 3.

Multiple is the number you come up with when you multiply that number by any whole number. For example, if your number is 3, you can multiply 3 by 50. The answer of 150 would be your foundation chain for a project that uses a multiple of 3.

If your pattern multiple is 3 + 1. Then you work the multiple as above and add 1 to that number. In the above example, the resulting foundation chain would be 151.

50 is just a sample number. You can use any number. The designer gives the multiple generally so that you can adapt the size of the project. You might like to make a scarf, lapghan, or afghan out of the specific stitch combinations of your pattern. For a scarf you might use 3 x 7 and start with a foundation chain of 21. For a lapghan you might go with 3 x 30 and start with 90. Those are just examples and not firm numbers. The numbers you choose will depend on other factors besides the multiple.