string[] listsymp = { "one", "two", "three" };
if (listsymp.All(CheckWord.ToLower().Contains))
 {
    richtextBox1.Text = CheckWord.ToString();
 }

if (CheckedSymptoms.ToLower().Contains("one") && CheckedSymptoms.ToLower().Contains("two") ||
    CheckedSymptoms.ToLower().Contains("two") && CheckedSymptoms.ToLower().Contains("three")