Regarding time complexity of multi-tape Turing machines
Multi-tape Turing Machine running in $O(f(n))$ can be simulated by a single-tape Turing machine running in $O(f^2(n))$.

Try writing a single-tape Turing machine that stores symbols from your $3$ tapes on its one tape, separated by some unique symbol not in $\Sigma$.



hint: For every step of your $k$-tape machine, a one-tape machine must first do at most $k\cdot n$ steps to find the position.