Hello, Voteary and welcome to knitting help.
You're right, one cant p3tog without decreasing. However, the (k1 p1 k1) all in next stitch will counterbalance this and get your stitich count back to 25.
Hope that helps and good luck with your pattern.
An English Knitter (nationality and method).
Feel free to call me either Clutter or Julie (my real name).
FO: Downton Hat
A sweater of a thousand yards begins with a single stitch. Lao-tzu's Granny