arXiv:2606.29351v2 Announce Type: replace Abstract: Operating envelopes (OEs) define the admissible active and reactive power injections of distributed energy resources while respecting distribution network limits. In unbalanced low-voltage feeders, OEs based only on voltage magnitude and thermal limits may overestimate available flexibility because they neglect power quality requirements. This paper develops a three-phase unbalanced AC optimal power flow framework to compute coupled active and reactive power OEs while enforcing limits on the voltage unbalance factor (VUF). It also proposes a fairness mechanism based on voltage sensitivities to allocate available flexibility among multiple photovoltaic (PV) units. The proposed mechanism is compared with lexicographic max--min fairness as a benchmark for equitable resource allocation. Case studies on unbalanced test feeders evaluate how VUF constraints change the feasible operating region of PV units. The results show how VUF constraints affect feasible flexibility and demonstrate that the proposed voltage sensitivity-based mechanism is nearly six times faster than lexicographic max-min fairness.