This work presents a novel methodology for speeding up the assembly of stiffness matrices for laminate composite 3D structures in the context of isogeometric and finite element discretizations. By splitting the involved terms into their in-plane and out-of-plane contributions, this method computes the problems's 3D stiffness matrix as a combination of 2D (in-plane) and 1D (out-of-plane) integrals. Therefore, the assembly's computational complexity is reduced to the one of a 2D problem. Additionally, the number of 2D integrals to be computed becomes independent of the number of material layers that constitute the laminated composite, it only depends on the number of different materials used (or different orientations of the same anisotropic material). Hence, when a high number of layers is present, the proposed technique reduces by orders of magnitude the computational time required to create the stiffness matrix with standard methods, being the resulting matrices identical up to machine precision. The predicted performance is illustrated through numerical experiments.