Jump inf assault into cover?

Here is the scenario...
10 fire warriors standing in difficult terrain an inch or so in
Boss Zag and his unit zoom in for an assault, survives over watch but only 3 of his boyz get base to base, the rest are waiting for their initiative step. On fire warriors initiative they leave the difficult terrain to engage as many boyz as possible. On boyz initiative step they end up not having to enter the difficult terrain
How many boyz need to make jump Inf dangerous terrain rolls? Just the three that actually entered it or all because the whole unit assaulted?

Depends if they used their jump packs to assault.

If they did use their jump packs to assault, only the ones who charged into the terrain in the first place, otherwise, non. Pile-in is a walk move.

So either 3 or 0, depending.

Did he get 3 hammer of wrath hits? If not, than 0 dangerous terrain rolls should be made.