Regular expressions with cycles
We can build it up from smaller pieces:

How can you describe sequences made up of only a's? Just $a^*$

How about sequences made up of only a's and b's? $(a|b)^*$
These can be thought of as turning the knob from A to B, then going back to A, etc..

Similarly for b's and c's:  $(b|c)^*$

Say we stay in positions A-B for a while, and then we want to go to C; this can be expressed as:
$(a|b)^*  (bb^*)  c$

Now let's say we stay in positions A-B for a while, and then we switch to C, and stay in B-C for a while. Then the rule means we must pass through b at least once:
${(a|b)^*  (bb^*)  (b|c)^*}$

Now what if we want to stay in A-B, then switch to C, stay in B-C, then switch back to A, and stay in A-B again? ${(a|b)^*  (bb^*)  (b|c)^* (bb^*) (a|b)^*}$

The pattern starts to appear. You can see that whenever we want to transition we need to stick a $bb^*$ in between.

To get full sequences, we could try:

$(\,  (a|b)^*(bb^*) \, | \, (c|b)^*(bb^*) \, )^*$
which you can check, obeys the transition rule. The only problem is that this assumes the sequence ends with b.  To fix that, we just include options to end at a or c; naming the above as
$$
L=(\,  (a|b)^*(bb^*) \, | \, (c|b)^*(bb^*) \, )^*,
$$
we arrive at an answer:  $L\, | \,La^* \,| \,Lc^*$