With the three-site Hubbard model and Floquet theorem, the dynamical behaviors of two electrons confined in a line shape coupled three quantum dots driven by an AC electric field are investigated. The Hamiltonian contains no spin-flip terms, so the singlet and triplet sub-spaces are completely decoupled and can be analyzed separately. The analysis indicates that the nine-dimensional triplet sub-spaces can be divided intos three-dimensional sub-spaces with the same direction of spin respectively, and the dynamical behaviors in each three-dimensional sub-space are similar to that of two-electron singlet state in coupled two-quantum dots driven by AC electric field. In the six-dimensional spin singlet sub-space, the calculations show that, for certain combination of frequency and strength of applied AC field, the tunnelling between various configurations can be strongly quenched. Specially, two electrons localized initially in one quantum dot can hold on for a span of time. Using the perturbation approach to solve the Floquet function leads to a detailed understanding of this effect.