We present far-UV observations of three peculiar magnetic cataclysmic variables (MCVs), BY Cam, V 1309 Ori and AE Aqr, obtained with the FUSE satellite. Previous IUE spectra of these three objects revealed quite unusual resonance lines compared to other MCVs: an intense NV line and a weak CIV line. The FUSE spectra of these sources exhibit broad OVI lines as well as a strong NIII line at 991Å, while the CIII 1175Å line is nearly absent. Photoionisation models fail to produce the observed line intensities. This confirms non-solar CNO abundances. We discuss possible origins for these peculiar abundances, including the signature of nova thermonuclear runaways (TNR) at the surface of the magnetized white dwarf. [ABSTRACT FROM AUTHOR]