Thermal copolymer fluctuations were explored in a three-component blend consisting of a critical (A/B) homopolymer blend and a symmetric A-B diblock copolymer using the technique of neutron small angle scattering. The copolymer has the function of an external nonordering field and thereby determines phase behavior as well as the regimes of 3d-Ising, isotropic Lifshitz, and Brasovskiî critical universality. It was found that the random phase approximation (RPA) does not correctly describe the copolymer structure function because of strong thermal fluctuations. On the other hand a weak coupling of copolymer and homopolymer was confirmed, consistent with predictions from RPA. Self-assembly of the copolymers was observed prior to the ordering of the "total" blend, e.g. inclusive of the homopolymers, into bicontinuous and lamellar ordered phases.