Ideas for levelling partial flooring for kitchen furniture
If it's not visible, just got for a manufactured board, e.g. plywood, OSB, etc.  For 3cm, you may be looking at two pieces stacked on top of each other.  If it's on concrete, install a moisture barrier first.  To secure the boards, I'd avoid the temptation to use a Ramset and permanently drive the names into the cement, since the next time the flooring is redone, you can take the opportunity to do it right and pull everything up and go wall to wall.  Instead, secure it with a few cut nails.  Again, this all assumes that none of this is visible.


(source: jmnails.com)