A novel complex-phase steel alloy with an unstable austenite ($\gamma$) phase was produced to study the deformation-induced martensitic transformation (DIMT) in steels. In-situ Electron Back-Scatter Diffraction (EBSD) was used to characterise the as-received condition of the alloy and evaluate $\alpha'$ martensite formation at higher applied stresses. The stability of the $\gamma$ grains against martensitic transformation was explored with in-situ Three-Dimensional X-ray Diffraction (3DXRD) by capturing over 900 $\varepsilon$ martensite formation events representing the early stages of the $\gamma \rightarrow \varepsilon \rightarrow \alpha'$ DIMT process. By connecting newly-formed $\varepsilon$ grains to their $\gamma$ parents using established orientation relationships, the influence of $\gamma$ microstructural properties such as grain size, orientation and neighbourhood configuration on $\gamma$ grain stability could be explored. Larger $\gamma$ grains were found to be less stable against transformation, forming $\varepsilon$ at lower applied strains. $\gamma$ grains with the $\{100\}$ axis oriented parallel to the loading axis were also less stable. Additionally, $\varepsilon$-forming $\gamma$ grains tended to possess a neighbourhood with a higher fraction of ferrite ($\alpha$) and/or $\alpha'$, indicating that the presence of these phases destabilises $\gamma$, promoting $\varepsilon$ formation. The efficacy of the minimum strain work variant model to predict $\varepsilon$ orientation variants was evaluated and found to work well for most grains. $\gamma$ grains where the prediction failed were found to have lower Type II stresses as measured with 3DXRD, indicating local Type III stress behaviour may sometimes modify the $\varepsilon$ variant that forms.