jmc

Let $d$ be the common difference in the first sequence. The first term in the first sequence is 0, so the terms in the first sequence are 0, $d$, $2d$, and so on.

We are told that the second term in the first sequence (namely $d$) is the sum of the first term in the first sequence (which is 0) and the first term of the second sequence, so the first term of the second sequence must be $d$.

We are also told that the third term in the first sequence (namely $2d$) is the sum of the second term in the first sequence (which is $d$) and the second term of the second sequence, so the second term of the second sequence must also be $d$.

The first two terms of the second sequence are both $d$, so all the terms must be $d$. We are told that the fifth term of the second sequence is 3, so $d=3$.

Finally, the fifth term of the first sequence is $4\cdot 3=\boxed{12}$.