First Generation
Developed by the mysterious Satoshi Nakamoto, Bitcoin and the simple alt-coins (like Litecoin) that were just cryptocurrencies. This went on from 2009 until roughly 2013/14.
Second Generation
The second generation of blockchain, led by the automation and the development of trusted code platforms like Ethereum and Hyperledger, introduced the concept of smart contracts and made possible the digital tokenization of physical assets. (considered 2014 until present).
Third Generation
Blockchain 3.0 would have to offer as significant of a change. Not just offering better capabilities, but doing so in such a way that they enable new markets and opportunities. Such as sharding to tackle scaling issues in order to cut down on cost and speed of transactions. These platforms also matured the distributed application capabilities of blockchain.