I know how to convert 0101 | 0111 in which the 0101 gives me 5, and 0111 gives me 7 So hexadecimal number is: 57

But how about 0011 | 1100 0011 gives me 3, but 1100 gives me 12, does it mean the hexadecimal number is 312?

Thanks!

-
No, $3C$, recall hex $A, B, C, D, E, F for 10$ through $15$. Regards – Amzoti Jan 15 '14 at 0:58
...and $12$ in decimal is C in hexa... :-) – Lucian Jan 15 '14 at 8:47

In hexadecimal you need 16 digits. This is usually achieved by using $0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F$. That is $F$ is fifteen and $10$ is sixteen.
When converting from binary to hexadecimal, each group of four binary digits becomes one hexadecimal digit. In the case you are asking, $1100_2$ corresponds to $C_{16}$.