Universal aspects of the critical Casimir force fc in wetting
films of 3He-4He mixtures near their bulk tricritical point are studied
within models of the corresponding universality class. fc for such films
of thickness L is calculated along isotherms at several fixed concentrations of
3He. Nonsymmetric boundary conditions impose nontrivial concentration profiles
leading to repulsive force which exhibit a rich crossover behavior between the tricritical point
and the line of critical points. The theoretical results agree semiquantitatively with published
experimental data and emphasize the importance of logarithmic corrections. The universal Casimir
amplitude is the amplitude of the leading behavior ~L-3(ln L)1/2
dominating the contribution ~L-3 whose amplitude is nonuniversal and which
is comparable with the background dispersion force contributions.