I removed architrave and most of the old plaster came away from behind the door frame so I bought a bag of bonding to fill up behind the door frame but as I can see the architrave in the other room I didn't know if I could use bonding at 3 inches so can I scrunch up newspaper in the gap before apply the bonding plaster. I did think of using expanding foam then going over with bonding plaster but as the the door frame is only nailed in to a block wall I didn't want the expanding foam pushing the frame off the wall.
Expanding foam won't push a door lining out, it would be a completely useless product if it had that capability. Post a photo or two, bonding shouldn't exceed roughly half inch thickness in one application.