  The Solar Orbiter mission, with an orbit outside the Sun Earth line and
leaving the ecliptic plane, opens up opportunities for the combined analysis of
measurements obtained by solar imagers and spectrometers. For the first time,
different space spectrometers will be located at wide angles to each other,
allowing three-dimensional (3D) spectroscopy of the solar atmosphere. The aim
of this work is to prepare the methodology to facilitate the reconstruction of
3D vector velocities from two stereoscopic LOS Doppler velocity measurements
using the Spectral Imaging of the Coronal Environment (SPICE) onboard the Solar
Orbiter and the near-Earth spectrometers, while widely separated in space. We
develop the methodology using the libraries designed earlier for the STEREO
mission but applied to spectroscopic data from the Hinode mission and the Solar
Dynamics Observatory. We use well-known methods of static and dynamic solar
rotation stereoscopy and the methods of EUV stereoscopic triangulation for
optically thin coronal EUV plasma emissions. We develop new algorithms using
analytical geometry in space to determine the 3D velocity in coronal loops. We
demonstrate our approach with the reconstruction of 3D velocity vectors in
plasma flows along "open" and "closed" magnetic loops. This technique will be
applied to an actual situation of two spacecraft at different separations with
spectrometers onboard (SPICE versus the Interface Region Imaging Spectrograph
(IRIS) and Hinode imaging spectrometer) during the Solar Orbiternominal phase.
We summarise how these observations can be coordinated.
