Do a row count check before you access a row. So basically you check if a row exist or not before accessing it.
string a , b, c;
if (objDt.Rows.Count > 0)
a = objDt.Rows[0][0].ToString();
if (objDt.Rows.Count > 1)
b = objDt.Rows[1][0].ToString();
if (objDt.Rows.Count > 2)
c = objDt.Rows[2][0].ToString();