This code not completely correct because 'a' has been not compared with 'c'.
The correct can be written this type
int a,b,c;
printf("enter 3 number");
scanf("%d%d%d",&a,&b,&c);
if (a<b){
if (b<c){
printf("the gretest number is %d ", c);
}else {
printf("the gretest number is %d ", b);
}
}else {
if(a>c){
printf("the gretest number is %d ",a);
else{
printf("the gretest number is %d ",c);
}
return 0;
}