boost::variant<T1*, T2, T3> Var;
 std::vector<Var> Vec;

std::vector<T2> T2Vec = ...(how to filter it from Vec)....

for (std::vector<Var>::const_iterator it = Vec.begin(); it != Vec.end(); ++it)
{
  if (it->which() == 1) T2Vec.push_back(boost::get<T2>(*it));
}

for (const auto& v : Vec)
{
  if (v.which() == 1) T2Vec.push_back(boost::get<T2>(v));
}