Var a, b, c, rez: real;
begin
write('Введите число А: ');
readln(a);
write('Введите число B: ');
readln(b);
write('Введите число C: ');
readln(c);
if (a >= 0) and (b >= 0) and (c >= 0) then rez := a + b + c;
if (a >= 0) and (b >= 0) then rez := a + b;
if (a >= 0) and (c >= 0) then rez := a + c;
if (b >= 0) and (c >= 0) then rez := b + c;
if (a <= 0) and (b <= 0) then rez := c;<br>if (b <= 0) and (c <= 0) then rez := a;<br>if (a <= 0) and (c <= 0) then rez := b;<br>if (a <= 0) and (b <= 0) and (c <= 0) then rez := 0;<br>writeln(rez);
end.
Громоздко, но, думаю, покатит