  Our main result states that when A, B, C are subsets of Z/NZ of respective
densities \alpha,\beta,\gamma, the sumset A + B + C contains an arithmetic
progression of length at least e^{c(\log N)^c} for densities \alpha > (\log
N)^{-2 + \epsilon} and \beta,\gamma > e^{-c(\log N)^c}, where c depends on
\epsilon. Previous results of this type required one set to have density at
least (\log N)^{-1 + o(1)}. Our argument relies on the method of Croot, Laba
and Sisask to establish a similar estimate for the sumset A + B and on the
recent advances on Roth's theorem by Sanders. We also obtain new estimates for
the analogous problem in the primes studied by Cui, Li and Xue.
