Before you start laying ply or hardboard over your existing floorboards ideally you should secure or fix any of the existing subfloors loose floorboards where necessary

You can use either ply or hardboard to level the subfloor they will both do the same job - please dont wet the hardboard or ply before its laid as all it will do is retain moisture as it is hygroscopic (like the laminate is) & then once your laminate is down the moisture will gradually raise upwards into the laminate causing the laminate to warp & bow @ the joints & also any moisture retained in the ply or hardboard could also lead to dampness in your existing subfloor leading to rotting of the original floor boards - believe me i have seen this happen far too many times when i used to handle all the diy complaints at my old place of work who provide flooring customer service for the majority of the uk's diy chains

Once the ply or hardboard has been fixed down a simple layer of 2mm foam underlay from somewhere like B&Q will suffice as underlay & the laminate can be laid on top