New trinuclear, oxo-centered, basic carboxylate compounds of transition metals. 3. Syntheses and x-ray studies of the trivanadium(III,III,III) and trivanadium(II,III,III) compounds [V3(.mu.3-O)(CH3CO2)6(CH3COOH)2(THF)]+[VCl4(CH3COOH)2]- and V3(.mu.3-O)(CF3CO2)6(THF)3 with "classical" triangular structures