Coud sombody explain to me what the differences in 7C/5C/3C exactly are.. I am confused...

I`m slow, dunnn laff

The basic difference is in the diameter of the cup. Go to ibowtie.com/tmptmpccharts.html. There`s a pretty decent mouthpiece chart there that gives measurements in mm and inches that will give you an idea of the differences. To elaborate just a little, in my opinion, the thicker ones lips, the larger diameter mpc he/she needs. But there are also other differences between the same size designations by differentbrands. For instance, I have an OLDS 7C and a Blessing 7C. The cup sizes are essentially the same, but the OLDS has a less rounded inner cup rim. By the way, I think Bach makes well over 100 different trumpet mouthpieces. If you can get hold of a Bach mpc chart (your local band supply store may have one on hand for reference), it`l make for a good half hour of interesting reading. Otherwise, just type "trumpet mouthpieces" in the url field of your browser and go surfing. You can keep busy reading about nothing but mouthpieces for quite a while.