Maybe if you can't fathom the precision required to irradiate the NIF hohlraum sufficiently isotropically to achieve ignition in the first place, you shouldn't be trying to answer this question.
There may be an engineering method to overcome this, but it would be way beyond the difficulty of getting that first pellet to ignite, which already is a bleeding edge technological development.
The same thing that stops you from igniting the initial pellet with the hohlraum - you don't have anything creating the kind of confinement necessary to keep the plasma together.
The only thing allowing the plasma to get hot enough for fusion is the initial velocity of the inward-spreading shockwave from the initial explosion of the outer shell of the pellet. As the velocity of this shockwave inevitably decreases, confinement is inevitably lost and the plasma dissipates and cools down.
Probably in principle you could use the energy of the first pellet's plasma to cause similar shockwaves in a second, larger pellet and so on, but that requires an entirely different geometry, its not just a matter of putting the second pellet close to the first one.
Between the primary fission bomb and the secondary fusion bomb there is a huge shield, so the shockwave of the first one hit's the second one at the same time everywhere, instead of hitting the top.
My guess is that to put a ternary fusion bomb you will need another even bigger shield, but IANANBS.