Приведем подобные члены: $$6t^3v^5-8t^5v^3+3t^3v^5=6t^3v^5+3t^3v^5-8t^5v^3=9t^3v^5-8t^5v^3$$
Ответ: $$9t^3v^5-8t^5v^3$$