The abstraction reactions, T + HD(DH) --> HT(DT) + D(H) and T + H-2(D-2) --> HT(DT) + H(D) were studied experimentally in liquid He-3-He-4 media at 1.3 K and theoretically using the gas-phase reaction model. The experimental reaction system has two characteristics; one is that the tritium atom (T) is produced from one of the constituents, He-3, through He-3(n, p)T nuclear reaction, and the other is that superfluid or normal-fluid reaction medium can be chosen arbitrarily by changing the composition-and temperature of the sample. The experimental isotope effect defined by {[HT]/[H-2]}/{[DT]/[D-2]} for the reactions T + H-2(D-2) --> HT(DT) + H(D) was found to be 158 in superfluid and 146 in normal-fluid solutions. The large isotope effects observed were qualitatively explained via quantum mechanical tunneling on the basis of the theoretical calculations of thermal rate constants for these reactions. In the T + HD(DH) --> HT(DT) + D(H) reactions, the experimental isotope effect was < 19.8. From the considerations on both the reaction processes based on the experimental results and the theoretical calculations, it has been suggested that the quantum mechanical tunneling abstraction through van der Waals complex plays a predominant role for both the reactions.