I think the writer is Japanese, actually. But here’s what you do:
You’re right, you have to keep the same number of sts. So you purl 3 sts together, which gives you 1 loop on the right needle. The trick is that you don’t take the 3 sts you purled together off of the left needle-- it’s kind of like you didn’t finish what you were doing. Now, bring the yarn over the right needle once (there is a video on this site for Yarn Over-- very simple to do). That’s your 2nd loop. Now, here’s why you left those 3 purled together sts on your left needle: you are again going to purl into those 3 original sts together. This gives you your 3rd loop and now you take off the 3 original purled sts off of the left needle. If you really like the pattern, then I’d go ahead and use it. I personally don’t like knitting or purling 3 sts together-- it’s tight to work the needle into them, and when you have to do it to all the sts on every other row for an entire scarf, well, that would drive me nuts. But if you like it, then make the scarf. If you decide not to, there are plenty of free scarf patterns on www.knittingpatterncentral.com.