program ct;
uses crt;
var n,t,a,b,c,min,max:integer;
begin clrscr;
   write('Nhap so n: '); readln(n);
     t:=n;
   while t<>0 do
     begin
        a:=a+t mod 10;
          t:=t div 10;
        b:=b+t mod 10;
          t:=t div 10;
        c:=c+t mod 10;
          t:=t div 10;
     end;
     max:=a; min:=a;
   if b>max then max:=b;
   if c>max then max:=c;
   if b<min then min:=b;
   if c<min then min:=c;
   writeln('So dao nguoc cua so ',n,' la: ',a,b,c);
   writeln('Chu so lon nhat trong ba so la: ',max);
   writeln('Chu so lon nho trong ba so la: ',min);
readln;
end.

Học tốt!

uses crt;
var n,max,min:string; i:longint;
begin
clrscr;
   write('Nhap vao so n:');readln(n); min:='9';
   write('So dao nguoc cua n la: ');
   for i:=3 downto 1 do
   begin if n[i]>max then max:=n[i];
   write(n[i]); if n[i]<min then min:=n[i]; end; writeln;
   writeln('So lon nhat trong ba so tao thanh n la: ',max);
   writeln('So be nhat trong ba so tao thanh n la: ',min);
readln
end.