The Excess-3 BCD system is formed by adding 0011 to each BCD value as in Table 2. For example, the decimal number 7, which is coded as 0111 in BCD, is coded as 0111+0011=1010 in Excess-3 BCD. 

Decimal Numerals
Binary Numerals
Excess-3
0
 0000
0011
1
 0001
0100
2
 0010
0101
3
 0011
0110
4
 0100
0111
5
 0101
1000
6
 0110
1001
7
 0111
1010
8
 1000
1011
9
 1001
1100

the one you gave is a logic diagram of BCD to excess-3 code converter. i already know this sir, but i wanted to know the logic circuit design for adding two different 4-bit excess 3 codes. anyways thnx for your reply. however after studying for like 1 and half hour i finally designed it and my instructor said it was right  i am uploading the picture for t he benefit of others, just in case if anyone needs it.