I have seen on blockchain.info that over 50% of the total hashrate is controlled by 3 mining pools. How do we know that they don’t come to an agreement to control the entire network Bitcoin allowing for example the double spending?
Older forum members know, it used to be one pool which had more than %50 hashrate, called Ghash.io. Nothing bad happened. If you try to do bad thing you'll harm Bitcoin and nobody use it anymore.
You don't need to worry, pools know what to do and what not.