Do you want to add 3' without support or are you building a new deck and want to know how far you can extend a 2x8 past the beam. For the first one you would have to put new 4x4 posts in the ground and new beams across and extend you deck that way. If the second one, it depends how how your deck is. If you have a 20' deck you could likely make 15' fully supported and 5' over hang provided that it's all one 2x8. On our deck it hangs over the beam about 3' with 2x6 joists and there is no extra movement.