  The aim of this work is to determine fundamental parameters of three Ap
stars, GO And (HD 4778), $\kappa$ Psc (HD 220825), and 84 UMa (HD 120198),
using spectroscopic techniques. By analysing these stars, we complete the
sample of Ap stars for which fundamental parameters have additionally been
derived by means of interferometry. This enables a cross-comparison of results
derived by direct and indirect methods. For all investigated stars, we
determined fundamental parameters and derived chemical abundances that are
typical for Ap stars. The abundances are mainly characterised by a gradual
increase of heavy element atmospheric abundances from an order of magnitude for
iron peak elements up to very significant excesses of 3-4 dex of the rare-earth
elements relative to the solar values. The only exception is Ba, whose
abundance is close to the solar abundance. There is also a significant He
deficiency in the atmospheres of HD 120198 and HD 220825, whereas the He
abundance in HD 4778 is close to the solar abundance. We do not find
significant Fe and Cr stratification. Using these abundances, we constructed
self-consistent atmospheric models for each star. The effect of the surface
chemical inhomogeneity on the derived fundamental parameters did not exceed
+/-100 K in effective temperature, which lies within the range of errors in
similar self-consistent analyses of Ap stars. Finally, we compared
spectroscopically derived effective temperatures, radii, and luminosity for 13
out of 14 Ap stars in a benchmark sample with the interferometric results.
While radii and luminosity agree within the quoted errors of both
determinations, the spectroscopic effective temperatures are higher than the
interferometric temperatures for stars with temperatures $T_{eff} >$ 9000 K.
The observed hydrogen line profiles favour the spectroscopically derived
temperatures.
