IF Statement multiple conditions, same statement

Check this for a more clustered way of checking conditions:

private bool IsColumn(string col, params string[] names) => names.Any(n => n == col);

usage:

private void CheckColumn()
{
     if(!IsColumn(ColName, "Column A", "Column B", "Column C"))
    {
     //not A B C column
    }

}

Use the && (and) operator in combination with the || (or) operator in a nested condition as so:

if (columnname != a 
  && columnname != b 
  && columnname != c
  && (checkbox.checked || columnname != A2))
{
   "statement 1"
}

Should do the trick.

if (columnname != a && columnname != b && columnname != c 
        && (columnname != A2 || checkbox.checked))
    {
      "statement 1"
    }

I always try to factor out complex boolean expressions into meaningful variables (you could probably think of better names based on what these columns are used for):

bool notColumnsABC = (columnname != a && columnname != b && columnname != c);
bool notColumnA2OrBoxIsChecked = ( columnname != A2 || checkbox.checked );

if (   notColumnsABC 
    && notColumnA2OrBoxIsChecked )
  {
      "statement 1"
  }