Magnetisation and longitudinal magnetoresistance of single-crystal samples of two compositions: stoichiometric compound U3P4 and its solid solution U-3(P,As)(4) (with As:P ratio close to 1) were measured in pulsed magnetic fields with strength up to 47 T, and in temperatures from 4 K up to few tens of kelvins above the Curie temperatures (135 K and 181 K for both compositions, respectively). Field was always applied along a hard magnetic direction [100]. Magnetisation experiments showed no sign of expected magnetic-moment-reorientation transition and data look quantitatively similar for both compositions. On the other hand, longitudinal magnetoresistance (MR) is remarkably different for each composition. Onset of the above mentioned transition can be held responsible for broad maxima observed in MR(B) curves for U-3(P,AS)(4). Values of magnetic field strength corresponding to these maxima follow a linear dependence on temperature. We assume that these features of MR(B) curves are due to progressive deformation of magnetic structure of U-3(P,As)(4) in high magnetic fields.