Diatomics-in-molecule (DIM) approach is compared with the point-charge approximation for transition moments in Rg₃⁺ and is shown to be its extension capable of accurately reproducing the direct ab initio results. Relevant polarisation corrections to the point-charge approximation are proposed. The developed procedure is applied to construction of transition moment and intensity (oscillator strength) 3D-surfaces of Ne₃⁺. They are used to evaluate the photoabsorption spectra of the bound species and of the Ne + Ne₂⁺ collisional system. The major absorption band of Ne₃⁺ is predicted to be red-shifted by 200 nm from that for the diatomic ion, and the absorption spectrum of Ne₂⁺ is found to be significantly affected in collisions with Ne.