Are GRB 980425 and GRB 031203 real outliers or twins of GRB 060218?

GRB 980425 and GRB 031203 are apparently two outliers with respect to the correlation between the isotropic equivalent energy Eiso emitted in the prompt radiation phase and the peak frequency Epeak of the spectrum in a νFν representation (the so-called Amati relation). We discuss if these two bursts are really different from the others or if their location in the Eiso–Epeak plane is the result of other effects, such as viewing them off-axis, or through a scattering screen, or a misinterpretation of their spectral properties. The latter case seems particularly interesting after GRB 060218 that, unlike GRB 031203 and GRB 980425, had a prompt emission detected both in hard and soft X-rays which lasted ∼2800 s. This allowed us to determine its Epeak and total emitted energy. Although it shares with GRB 031203 the total energetics, it is not an outlier with respect to the Amati correlation. We then investigate if a hard-to-soft spectral evolution in GRB 031203 and GRB 980425, consistent with all the observed properties, can give rise to a time-integrated spectrum with an Epeak consistent with the Amati relation.