We can choose a seat on the left for for B in 7 ways.

That leaves 13 seats (6 on the left and 7 on the right)

For each of the 7 ways we can choose a seat for B, we

can choose a seat for A in 13 ways. That's 7×13 or 91 ways
to seat B and A.

That leaves 12 vacant seats.

For each of those 91 ways we can seat B and A, we can seat C
in any one of those remaining 12 seats.

So that's a total of 12×91 = 1092 ways.

Edwin