© 2018 Author(s). It has been shown that Fe2MnSi exhibits a ferromagnetic transition at Tc and an antiferromagnetic transition at a lower temperature, TA, to a phase referred to as the AF phase. In a recent study on Fe1.3Mn1.7Si, another antiferromagnetic transition at a temperature lower than TA, defined as TA2, was found, with the phase below TA2 referred to as the AF2 phase. In this study, magnetic properties of Fe3-xMnxSi are investigated for an x range of 1.65 ≤ x ≤ 1.85 in order to study theses transitions with varying x. For x ≥ 1.75, a transition characterized by a rather rapid decrease in the temperature dependence of magnetization is observed at a temperature lower than TA at fields higher than ∼2 T. This implies the AF2-AF transition exists for x ≥ 1.75 as for x = 1.7. The magnetic field where the AF2-AF transition occurs increases with x, whereas at lower fields TA2 does not depend strongly on x. Meanwhile, at near zero field, the ferromagnetic transition and spontaneous magnetization disappears for x ≥ 1.75, in contrast to the case for x = 1.7. This implies that the transition from paramagnetism directly to the AF2 phase occurs at low fields. These results are summarized in the B-T magnetic phase diagram.