  Based on a theoretical selection of pulsars as candidates for detection at
X-ray energies, we present an analysis of archival X-ray observations done with
Chandra and XMM-Newton of PSR J1747-2958 (the pulsar in the "Mouse" nebula),
PSR J2021+3651 (the pulsar in the "Dragonfly" nebula), and PSR J1826-1256.
X-ray pulsations from PSR J1747-2958 and PSR J1826-1256 are detected for the
first time, and a previously reported hint of an X-ray pulsation from PSR
J2021+3651 is confirmed with a higher significance. We analyze these pulsars'
spectra in regards to the theoretically predicted energy distribution, finding
a remarkable agreement, and provide here a refined calculation of the model
parameters taking into account the newly derived X-ray spectral data.
