Abstract. We construct a fully discrete numerical scheme for three-dimensional incompressible fluids with mass diffusion (in density-velocity-pressure formulation), also called the KazhikhovSmagulov model. We will prove conditional stability and convergence, by using at most C 0 -finite elements, although the density of the limit problem will have H 2 -regularity. The key idea of our argument is first to obtain pointwise estimates for the discrete density by imposing the constraint lim (h,k)→0 h/k = 0 on the time and space parameters (k, h). Afterwards, under the same constraint on the parameters, strong estimates for the discrete density in l ∞ (H 1 ) and for the discrete Laplacian of the density in l 2 (L 2 ) are obtained. From here, the compactness and convergence of the scheme can be concluded with similar arguments as we used in [Math. Comp., to appear], where a different scheme is studied for two-dimensional domains which is unconditionally stable and convergent. Moreover, we study the asymptotic behavior of the numerical scheme as the diffusion parameter λ goes to zero, obtaining convergence as (k, h, λ) → 0 towards a weak solution of the density-dependent Navier-Stokes system provided that the constraint lim (λ,h,k)→0 h/(λ 2 k) = 0 on (h, k, λ) is satisfied.