About MUX circuits

[speedy94519]

Hello, Im having trouble deciding what type of MUX circuit I should use, for example 2:1,4:1, etc. I was given a problem about designing a circuit on a block diagram level that performs the following math operations:
RES = A + A
RES = A + C
RES = A + B
RES = B + C
The inputs A,B, and C, as well as the out put (RES), have 12-bit values. Im suppose to assume that their will be no issues or problems with carry out values. As you can see, depending on two select inputs, the single output will result from one of those operations and it does not matter which select values select which operation, I just have to make sure the combinations associated with the select inputs can generate each of the following operations.
MY solution so far:
The first thing that came to my mind was of course the MUX circuit since it has the ability to output the desired input. But then as I began to examine the MUX circuit more closely, I realized that I cant have just one MUX circuit because I cant have three inputs just go into a MUX circuit because they only exist as a binary representation: 2:1, 4:1, 8:1,etc. I began to brain storm a couple of modules that will solve this but Im not sure. I get the fact that I have to use a RCA circuit to add the final two outputs that the MUX circuits will produce, but im not sure how the MUX circuit is going to produce this. Can anyone give me some suggestions? Thank you, I greatly appreciate your help.

[apines]

Mux circuits always come with input value that are powers of two, but you can define that some input entrances are blocked, meaning that no value will come of them.