  In the context of the high resolution, high signal-to-noise ratio, high
sensitivity, spectropolarimetric survey BritePol, which complements
observations by the BRITE constellation of nanosatellites for asteroseismology,
we are looking for and measuring the magnetic field of all stars brighter than
V=4. In this paper, we present circularly polarised spectra obtained with
HarpsPol at ESO in La Silla (Chile) and ESPaDOnS at CFHT (Hawaii) for 3 hot
evolved stars: $\iota$ Car, HR 3890, and $\epsilon$ CMa. We detected a magnetic
field in all 3 stars. Each star has been observed several times to confirm the
magnetic detections and check for variability. The stellar parameters of the 3
objects were determined and their evolutionary status was ascertained employing
evolution models computed with the Geneva code. $\epsilon$ CMa was already
known and is confirmed to be magnetic, but our modeling indicates that it is
located near the end of the main sequence, i.e. it is still in a core hydrogen
burning phase. $\iota$ Car and HR 3890 are the first discoveries of magnetic
hot supergiants located well after the end of the main sequence on the HR
diagram. These stars are probably the descendants of main sequence magnetic
massive stars. Their current field strength (a few G) is compatible with
magnetic flux conservation during stellar evolution. These results provide
observational constraints for the development of future evolutionary models of
hot stars including a fossil magnetic field.
