when playing a C, you don't press down any valves, so using the tuning slides wont do anything. The slide attached to the 3rd valve will only do something to the notes that use the 3rd valve (low D and C# for example). the slide on the 1st valve (if you have one) does only get used very rarely on my trumpet. I usually only use the one on the 3rd slide, and mosly on the notes below the staff.
or are we talking about the slide that goes before the valves, just after the mouthpiece? In that case you'll just have to work on you embouchure. once your lips are stronger, you can get out the right notes the way you're supposed to. In general, you shouldn't touch the slide after using it. One position of the slide is all you need during pretty much any piece. The distance you have to pull it out may change from day to day, or by temperature differences. make sure that you're warmed up before tuning the thing.