The given sequence is $3, 6, 9, 12, \dots$. This is an arithmetic sequence where the first term $a_1 = 3$ and the common difference $d = 3$.

To find the $n$-th term of an arithmetic sequence, the formula is:

$a_n = a_1 + (n-1) \cdot d$

Substitute the values for $a_1 = 3$, $d = 3$, and $n = 80$:

$a_{80} = 3 + (80-1) \cdot 3 = 3 + 79 \cdot 3 = 3 + 237 = 240$

Thus, the 80th term of the sequence is 240.

Would you like a more detailed explanation or have any questions?

Here are 5 related questions you might explore:

1. What is the sum of the first 80 terms of this sequence?
2. What is the general formula for the sum of an arithmetic sequence?
3. How would you find the 50th term in a sequence with a common difference of 5?
4. If the sequence starts at 5 instead of 3, what would be the 80th term?
5. What is the difference between an arithmetic sequence and a geometric sequence?

Tip: In an arithmetic sequence, the difference between consecutive terms is always constant.