Investigations are still ongoing.
It doesn't take 3 years to find proof of an assassination when you've been ranting about it for 5 years previously. Smoleńsk is and was just a political weapon, as witnessed by the way that it gets dropped by PiS before elections. If they really intended on finding out any truth, then they would have put together a commission made up of genuine international experts, not just a bunch of people with zero experience in investigating disasters.
It doesn't help when the Russians won't release the black boxes or the wreckage.
They don't need to release the black boxes. One-to-one copies were made after the disaster by the Poles, and PAF-101 also had a special Polish black box that could only be decoded by the Polish military.
The wreckage? What makes you think that Poland actually wants it back? After all, Kaczyński personally ordered them to drop any talk of a case at the international courts. Why would he do that if they actually wanted it back?
Incidentally, Macierewicz's sub commission didn't actually bother to visit the crash site or the wreckage. Is it because they also know that their conspiracy theories are just propaganda, and that there's little point in going to a crap Russian city when the facts are already known?