Addition of 3 bits using Full Adder when the initial carry bit is 0
A full adder adds three bits and outputs a sum and a carry. If you want to add three bits, you just connect them to the inputs x, y, and carry of the full adder. 

If for some reason there is a requirement that the carry input of the first full adder must be zero, then you leave that first full adder totally unconnected and connect to the three inputs of the second full adder.