You'll need to elaborate further.
Even if one company with 1 000 000 L3's is running a mining operation and only one other person mines with a 1070, the nodes are still verifying transactions in a decentralised manner. The only way the company can manipulate anything other than what the whitepaper and algorithm outline is if it creates a duplicate copy of Bitcoin Core - a hardfork...but then we're no longer talking about bitcoin.
Centralisation becomes an issue when you give a single entity control over the source code of the bitcoin reference client that all nodes need to run <- it's open source, so will never happen.
If you do not understand this concept, I'd recommend you steer clear of topics dealing with bitcoin's decentralised nature.