In SRP-6 $B$ is calculated as $B=kv+ g^b, k=3$. What is the purpose of $k$, and why was it fixed as $3$?
(In SRP-6a, this value $3$ is replaced by $k = H(N,g)$, but this question is about SRP-6.)
The purpose is to prevent a two-for-one guessing attack, where an active adversary, impersonating the server, can test two password guesses per attempt. The attack and why the multiplier prevents it is described in Section 2 of the SRP-6 paper (ps). (According to MacKenzie, it was discovered by Bleichenbacher.)
In brief, the attack goes like this:
If the attacker does not know the discrete logarithm of $k$, i.e. the number $l$ for which $g^l = k$, they cannot try two guesses at once with the version 6 protocol where $v$ is multiplied by $k$.
The paper shows why $k=3$ is a safe choice for generic $g$ and $N$.
(The hashed $k$ fixes it for maliciously chosen $g$ and $N$ as well.)