  We perform the first high-sensitivity soft X-ray long-term monitoring with
Swift/XRT of three relatively unexplored Supergiant Fast X-ray Transients
(SFXTs), IGR J08408-4503, IGR J16328-4726, and IGR J16465-4507, whose hard
X-ray duty cycles are the lowest measured among the SFXT sample, and compare
their properties with those of the prototypical SFXTs. The behaviour of J08408
and J16328 resembles that of other SFXTs, and it is characterized by a
relatively high inactivity duty cycle (IDC) and pronounced dynamic range (DR)
in the X-ray luminosity. Like the SFXT prototypes, J08408 shows two distinct
populations of flares, the first one associated with the brightest outbursts
($L_{\rm X}\gtrsim 10^{35-36}$ erg s$^{-1}$), the second one comprising less
bright events with $L_{\rm X}\lesssim$10$^{35}$ erg s$^{-1}$. This
double-peaked distribution seems to be a ubiquitous feature of the extreme
SFXTs. The lower DR of J16328 suggests it is an intermediate SFXT. We find
J16465 is characterized by IDC$\sim$5% and DR$\sim$40, reminiscent of classical
supergiant HMXBs. The duty cycles measured with XRT are found to be comparable
with those reported previously by BAT and INTEGRAL, when the higher limiting
sensitivities of these instruments are taken into account and sufficiently long
observational campaigns are available. We prove that no clear correlation
exists between the duty cycles of the SFXTs and their orbital periods, which
makes it difficult to interpret the SFXT peculiar variability by only using
arguments related to the properties of supergiant star winds. Our findings
favour the idea that a correct interpretation of the SFXT phenomenology
requires a mechanism to strongly reduce the mass accretion rate onto the
compact object during most of its orbit around the companion, as proposed in a
number of theoretical works. [Abridged]
