CO (cast on) in multiples of 3 means that you would cast on 3, 6, 9, 12, etc., ANY multiple of 3 would work with that particular stitch pattern. It is not, however, the same as multiple of 2+1. An example of that would be 5. Two stitches, times two (multiple of two) plus 1 equals 5. Another example would be 7 (2x3+1) or 9 (2x4+1) etc.
*k2, (yo, pull the last st over the yo) 4 times; rep from *
First off, do you know what a ‘yo’ is? If not, I’ll direct you to the abbreviations page where you will find a video demonstrating a yarn over (yo). Once that’s clear, you will start off your row with a K2, which you understand, then a yo onto the right needle, then the stitch that is immediately next to the yo will be lifted over the yo and off the needle. You will repeat only the part within the brackets, a total of 4 times, and then because there’s a ‘rep from *’, you will return back to the beginning of the instruction and do the same thing over again, as many times as it takes you to get to the end of your row.