If you want to see the result in C1, write =B1 + B2 - B3 inside the cell C1 and press enter. Writing formula means it is dynamic, whenever you change the value of B1, B2 or B3; you see the change in result in C1.
To add B1 & B2 we have to write =sum(B1:B2) in any cell we like. And similarly =sub(D4:B3). To learn microsoft excel please refer to some nice book based on basic courses available. There you can find all formulae for calculation. Here D4 is a select cell were you have added (B1:B2).