http://en.wikipedia.org/wiki/Excess-3
from this info you can develop a truth table for your excess-3 inputs, with 5 output columns also in the truth table, one for each digit of the BCD number.
Something like: A-D = inputs, Fs = outputs
A B C D F4 F3 F2 F1 F0
0 0 0 0 1 1 1 0 1
0 0 0 1 1 1 1 1 0
0 0 1 0 1 1 1 1 1
0 0 1 1 0 0 0 0 0
0 1 0 0 0 0 0 0 1
etc etc
1 1 1 1 0 1 1 0 0
In this case I've put in the 5th output (F4) as a sign indicator, where 1 signifies the BCD value is negative. I've used 2s complement form to determine BCD for the negative excess-3 values.
A = MSB of input, D = LSB of input (Excess-3)
F4 = MSB of output, F0 = LSB of output (BCD)
From this truth table you can write up a K-map for each output F4 through F0. Simplest way is probably just get sum of products form for each output and do a quick AND/OR implementation for each output line, given the 4 inputs. If you want to simplify you can collect like terms as you desire.
If you don't know how to turn a K-map into a circuit you should probably revise that a lot. It's quite simple and you should do lots of practice to make it easy for yourself. It's too detailed for me to bother going into it here. Try your school library for a suitable textbook or just go to class more often.
EDIT: If either of my method or interpretation are wrong please excuse me! only in 1st year electrical eng course and it's late at night right now.
