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).
FO: Downton Hat
Call me either Clutter or Julie (my real name), if you find it easier to type than Dclutterchique.