Please give me the proof that for a formal space Massey triple products vanish.



Formal Space and Rational Homotopy Theory
This is a very special case of a very general result. A quasi-isomorphism of DGAs identifies Massey products (not just triple products but matrix Massey products of all sizes). See for example Theorem 1.5 of ``Matric Massey products'', <http://www.math.uchicago.edu/~may/PAPERS/8.pdf>. But of course you must interpret your statement modulo indeterminacy.


