This could be done similar to how Rory suggested. I'd do this with an oscillator with an amp of 50 (it will scale -50 to 50), and then add it to 50, giving you a scale of 0 to 100... and set the frequency of the oscillator as 1/3, aka once every three seconds.
kVol oscil 50, 1/3
chnset kVol+50, "volumeSlider"
This is also pseudocode, so I may have messed something up... but as long as your widget also shares the channel name "volumeSlider", it should continuously move between 0 and 100 once every 3 seconds.
Think of cabbage as a way to create widgets that correspond to channels which can be accessed in csound, but then csound is the actual language that does the math and processing aspects behind the scenes.
Hopefully this helps!