Hi,
I think what you would want to ask is that how could you implement addition of three 4-bit binary using full adders and half adders.
First, you have to know that a full adder has 3 inputs and two outputs. While a half-adder has 2 inputs with also two outputs. The inputs correspond to the binary numbers you are adding and outputs corresponds to the sum and carry out.
Let us assume that the first 4-bit number is A3 A2 A1 A0 with A3 as the most significant bit and A0 as the least significant bit. The next 4-bit number is B3 B2 B1 B0 and the third is C3 C2 C1 C0.
Align the three 4-bit number you are to add:
A3 A2 A1 A0
+ B3 B2 B1 B0
+ C3 C2 C1 C0
Cout S3 S2 S1 S0
I recommend you to assume the values of your 4 bit binary numbers to be 1111. In this way, the maximum number of carry outs will be accounted for the design.