<*LIMIT>:VAR(DEVICE).IFEQ(Mobile).THEN(3).ELSE(6);<*/LIMIT>

(remove the stars)

Oh man, you should have just said what you wanted to do. 😅

First of all, this cannot be done using media queries (not the way you want).

In the Post Lister Configs, type the following:

<LIMIT>:VAR(DEVICE).IFEQ(Mobile).THEN(3).ELSE(6);</LIMIT>

And that's it. Let me know if it worked or not.