"Do three ladders, for a total of 18 repetitions, the first week; add a ladder the next week and a ladder the week after. Five ladders, 30 total reps. You will stay with five ladders from now on."
= only 3 ladders in week 1. Using the rules established by the book (same number on ladders, but reduced number of rungs for light & medium) we end up with 3 singles and (1,2)x3 for the rest of the first week.
That's how most people here interpret it.
After the part I quoted Pavel goes on to explain how to progress further (adding rungs). He then goes on to explain how to handle the light and medium days (-2 rungs on light & -1 rung on medium day) with a relatively long example.
After that comes the part, that you quoted:
"If you are climbing (1, 2, 3) ladders on the heavy day, do 5 singles on the light day and 5 x (1, 2) on the medium day"
This quote is introduced by "Another example:", so it's just a second, shorter and easier to understand example to show how the heavy-light-medium schedule works.
The example is used in the context, that we already work with 5 ladders.
In the end it wouldn't really matter whether you do it the way you interpret it or use the other way. 5 singles instead of 3 won't kill you
neither will 15 total reps instead of 9 on your medium day in week 1, the total rep count is way too low for that.
Please note that the chart I posted is one way to do the RoP. You work your way up from 3 to 5 ladders at the beginning and then add a rung to the heavy day each week. This way you end up with a 13 week cycle that's progressive, but sustainable.
If you go 100% by the book you'd have to test your limit on every heavy day. By doing that your cycle can be shorter. In the long run, though, it might not be so sustainable.