Answer
See below
Work Step by Step
Consider $V_1 \cong V_2\\
V_2 \cong V_3$
then there exist isomorphisms $T_1:V_1 \rightarrow V_2\\
T_2: V_2 \rightarrow V_3$
We have: $T_2T_1:V_1 \rightarrow V_3$
Since $T_1 \& T_2$ are both one-to-one and onto, so $T_2T_1$ is also one-to-one and onto. Consequently, $T_2T_1$ is also an isomorphism.
Hence, $V_1 \cong V_3$
