Работа со строками

program prog;
 
const
  letters: set of char = ['a'..'z', 'A'..'Z'];
 
var
  ch: char;
  s: string;
  i: integer;
  A, B, C, D: set of char;
begin
  write('input s1: ');
  readLn(s);
  A:= [];
  for i:= 1 to Length(s) do
    if s[i] in letters then A:= A + [ s[i] ];
 
  write('input s2: ');
  readLn(s);
  B:= [];
  for i:= 1 to Length(s) do
    if s[i] in letters then B:= B + [ s[i] ];
 
  write('input s3: ');
  readLn(s);
  C:= [];
  for i:= 1 to Length(s) do
    if s[i] in letters then C:= C + [ s[i] ];
 
  D:= A * B * C;
 
  write('result: ');
  for ch:= Low(char) to High(char) do
    if ch in D then write(ch, ' ');
  writeLn;
 
  readLn;
end.

program prog;
 
const
  letters: set of char = ['a'..'z', 'A'..'Z'];
 
var
  i: integer;
  s1, s2, s3, res: string;
begin
  write('input s1: ');
  readLn(s1);
 
  write('input s2: ');
  readLn(s2);
 
  write('input s3: ');
  readLn(s3);
 
  res:= '';
  for i:= 1 to length(s1) do begin
    if s1[i] in letters then
      if (pos(s1[i], s2) <> 0) and (pos(s1[i], s3) <> 0) then
        if pos(s1[i], res) = 0 then res:= res + s1[i];
  end;
 
  writeLn(res);
  readLn;
end.