Let F be a cuspidal eigenform of even weight and trivial nebentypus, let p be a prime not dividing the level of F , and let ρ F be the p-adic Galois representation attached to F . Assume that the L-function attached to Sym 3 (ρ F ) vanishes to odd order at its central point. Then under some mild hypotheses, and conditional on certain consequences of Arthur's conjectures, we construct a nontrivial element in the Bloch-Kato Selmer group of an appropriate twist of Sym 3 (ρ F ), in accordance with the Bloch-Kato conjectures.Our technique is based on the method of Skinner and Urban. We construct a class in the appropriate Selmer group by p-adically deforming Eisenstein series for the exceptional group G 2 in a generically cuspidal family and then studying a lattice in the corresponding family of G 2 -Galois representations. We also make a detailed study of the specific conjectures used and explain how one might try to prove them.