In this paper, we consider the estimation problem of individual weights of three objects. For the estimation we use the chemical balance weighing design and the criterion of D-optimality. We assume that the error terms ε i , i = 1, 2, . . . , n, are a first-order autoregressive process. This assumption implies that the covariance matrix of errors depends on the known parameter ρ. We present the chemical balance weighing design matrix X and we prove that this design is D-optimal in certain classes of designs for ρ ∈ [0, 1) and it is also D-optimal in the class of designs with the design matrix X ∈ M n×3 (±1) for some ρ ≥ 0. We prove also the necessary and sufficient conditions under which the design is D-optimal in the class of designs M n×3 (±1), if ρ ∈ [0, 1/(n − 2)). We present also the matrix of the D-optimal factorial design with 3 two-level factors.