The magnetic properties of a nanoscale systemare inextricably linked to its local environment. In adatoms onsurfaces and inorganic layered structures, the exchangeinteractions result from the relative lattice positions, layerthicknesses, and other environmental parameters. Here, wereport on a sample-dependent sign inversion of the magneticexchange coupling between the three unpaired spins of anorganic triradical molecule embedded in a three-terminaldevice. This ferro-to-antiferromagnetic transition is due tostructural distortions and results in a high-to-low spin ground-state change in a molecule traditionally considered to be a robust high-spin quartet. Moreover, theflexibility of the moleculeyields an in situ electric tunability of the exchange coupling via the gate electrode. Thesefindings open a route to the controlledreversal of the magnetic states in organic molecule-based nanodevices by mechanical means, electrical gating, or chemicaltailoring