Let A, B and C be the witnesses.  
And x denotes the mugger. 
A: S=x 
B: S=x OR C=x 
C: C=x 
 
IF() 
A was bribed; 
=> S=x & D or C was saved 
 
B was bribed; 
=> doesnt make sense  
 
C was bribed; 
=> C=x & D or S was saved 
 
Ignore All the "2 witnesses briberies", 
they dont make sense.  
 
But!  
IF all the 3 witnesses were bribed, the TRUE statements should be: 
A' : D=x OR C=x 
B' : D=x 
C' : D=x 
Ahaaa! Y u lil brat. . .! 
^^^See how D tried to save himself from all sides  
 
Doubtful Des is the mugger! 
Since the logic isnt always applicable in modern life, where one may lie by speaking the truth,..  
That simply puts him to a question... 
"Will he still try to save himself with expensive lawyers and experts?" 
 
0_x

IF all the 3 witnesses were bribed, the TRUE statements should be:  
A' : D=x OR C=x  
B' : D=x  
C' : D=x  
??? Why? 
C' : D=X OR S=X