Ответ:
PascalABC.NET 3.5.1 + .NET Framework 4.7.1
#1
begin
var A := ReadSeqInteger('enter seqeunce: ',ReadInteger('seq length: '));
A.Where(p -> p mod 3 = 0).Count.Println;
end.
#2
begin
var A := ReadSeqInteger('enter seqeunce: ',ReadInteger('seq length: '));
A.Where(p -> (p>=100) and (p <= 999)).ToList.Println;</strong>
end.
#3
begin
Range(1,10).Select(p -> p * p).sum.Println;
end.
Объяснение: