Theres a real easy and cheap solution to this. Rather than plaster the walls to get a flat level finish simply level the walls by putting a whole new sheet of plater board over the entire wall to get it level, cieling to behind skirting. You dont need to plaster the wall. This works perfectly if your tiling the entire wall. Becasue you have a bowing to a degree of 3cm you may find placing sheets of plaster board along the affected areas helps level the walls. Its going to be hidden buy the tiles you lay so it doesnt matter so much if it looks rough.
If your not doing the entire wall you may find applying more tile grout to the tiles you place helps level the walls, 3cm is a big difference to build up on though! Are the walls still quite solid in the bowed areas? if the walls seem quite bouncy, and push back into shape after you press them? this could be the plasters blown and wouldnt really provide a sound base on which to lay tiles anyway. They will most likely bring the section of wall down with the additional wieght of the tiles....hope all this makes sense and helps you out...let me know how you get on. Cheers
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