The high uptake of distributed energy resources (DER), particularly solar photovoltaics (PV), may cause the technical limits of electricity distribution networks to be breached. It is therefore necessary to determine the network DER hosting capacity, which ensures the technical limits are not violated. Above the network DER hosting capacity, it is then possible to investigate constraint alleviating approaches such as network upgrades or active network management. In this paper, we propose a DER hosting capacity envelope which contains lower (worst-case) and upper (best-case) bounds of the DER hosting capacity of unbalanced three-phase distribution systems. We obtain these bounds using two optimization-based hosting capacity assessment models. These two models are complemented by another hosting capacity model which takes the fairness concept into account. We also show how this latter, fair hosting capacity assessment model, can be calculated analytically using the Jacobian inverse matrix at a given operating point. The proposed hosting capacity assessment models are used to calculate the DER hosting capacity of the IEEE 37-node MV and IEEE 906-node LV unbalanced distribution networks under various load and generation scenarios.