  We present two years of Neutron star Interior Composition Explorer (NICER)
X-ray observations of three energetic rotation-powered millisecond pulsars
(MSPs): PSRs B1937+21, B1821-24, and J0218+4232. We fit Gaussians and
Lorentzians to the pulse profiles for different energy sub-bands of the soft
X-ray regime to measure the energy dependence of pulse separation and width. We
find that the separation between pulse components of PSR J0218+4232 decreases
with increasing energy at $\gt 3\sigma$ confidence. The 95% upper limit on
pulse separation evolution for PSRs B1937+21 and B1821-24 is less than 2
milliperiods per keV. Our phase-resolved spectral results provide updated
constraints on the non-thermal X-ray emission of these three pulsars. The
photon indices of the modeled X-ray emission spectra for each pulse component
of PSR B1937+21 are inconsistent with each other at the 90% confidence level,
suggesting different emission origins for each pulse. We find that the PSR
B1821-24 and PSR J0218+4232 emission spectra are invariant with phase at the
90% confidence level. We describe the implications of our profile and spectral
results in the context of equatorial current sheet emission models for these
three MSPs with non-thermal, magnetospheric X-ray emission.
