Is this big O notation format correct? $3^n = 2^{(O(n))}$
Sure, that is a correct notation. It is also a correct statement.
Essentially it is a compact way to say the following:

$\exists c&gt;0, \exists n_0 \ge 0 : \forall n&gt;n_0, 3^n \le 2^{c n} $.

You can pick any $c  \ge \log_2 3$ and $n_0 = 0$.