  In this paper, closed forms of the summation formulas for generalized
Tribonacci numbers are presented. Then, some previous results are recovered as
particular cases of the present results. As special cases, we give summation
formulas of Tribonacci, Tribonacci-Lucas, Padovan, Perrin, Narayana and some
other third order linear recurrance sequences. All the summing formulas of well
known recurrence sequences which we deal with are linear except the cases
Pell-Padovan and Padovan-Perrin.
