I'm assuming you mean that you retune ALL the strings from the standard "E", drop down to "D" and drop even further to "C". So, if you drop to "C" tuning you have gone down 2 whole steps (4 half steps) which would be the same # of frets you would move. For example, standard tuning basic E chord becomes a D chord in D tuning becomes a C chord in C tuning relative to pitch. Again, basic tuning A barre chord at fifth fret becomes G in drop D becomes F in drop C. So, to play A barre chord in drop C tuning, you would move UP four frets to what would in normal tuning be a C# barre chord. So, you move UP the same number of half-steps you tune down. I hope I haven't confused you as much as I've confused myself.