System > Every 2 seconds > add 2 myvariable
subevent ________ >(x)myvariable < 0 > System Subtract 3 from myvariable
x is for inverted. Every 2 seconds ( if the myvariable is not less than or equal to 0) , subtract 3 from myvariable
There's a probably a few ways to make this better or implement differently but, depends on your use. Hope this helps or gets you in the right direction.