Liu, Jian Ya et Zhan, Tao, The Exceptional Set in Huas Theorem for Three Squares of Primes, Acta mathematica Sinica. English series (Print) , 21(2), 2004, pp. 335-350
It is proved that with at most O(N 11/12*) exceptions, all positive integers n N satisfying some necessary congruence conditions are the sum of three squares of primes. This improves substantially the previous results in this direction.