This work presents an investigation about the type 3 non-uniqueness of the International Temperature Scale of 1990 (ITS-90) between the aluminium (660°C) and silver (962°C) freezing points, carried out at the Italian National Institute of Metrology (INRiM) within the framework of the EMRP—JRP SIB10 NOTED (Novel Techniques for Traceable Temperature Dissemination). For this purpose six High Temperature Standard Platinum Resistance Thermometers (HTSPRTs) from different manufacturers were calibrated at the ITS-90 fixed points, then intercompared in a sodium Gas Controlled Heat Pipe (GCHP) at 25K intervals between the aforementioned fixed points; a temperature uniformity and stability within ±0.5mK was achieved in the whole range between 660°C and 962°C. This paper presents the results of type 3 non-uniqueness analysis of these test HTSPRTs, together with problems deriving from their stability, which is the largest source of uncertainty in this work.