If 𝑎 = 3 a=3 and 𝑑 = 0 d=0, what is the 5th number in the arithmetic series?

If the common difference $d = 0$ , the sequence is constant.

General formula for an arithmetic sequence:
$a_n = a + (n - 1)d$

For $d = 0$
$a_n = 3 + (n - 1) times 0 = 3$

So, the 5th number $a_5 = 3$