EP3 7.4.11
The mass parameter assignments are not correct.
You need these to get the 2 mass 3 spring system:
  m1 = 1, m2 = 2/3, k1 = 32, k2 = 8, k3 = 32
A weak spring in the middle, and a slightly bigger mass on the left.

When you find the natural frequencies, you see why the driving frequency of 7 was chosen. In fact it is close to both (the average!) and if you resolve this problem with the system at rest at equilibrium, you see the beating clearly that results.

Curious?

ep3_7-4_1-7.mw