Bitcoin was Blockchain 1.0 and the decentralized Ethereum is Blockchain 2.0 and Blockchain 3.0 could be Direct Acyclic Graph (DAG).

In mathematics and computer science, a directed acyclic graph, is a finite directed graph with no directed cycles. It consists of finitely many vertices and edges, with each edge directed from one vertex to another, such that there is no way to start at any vertex v and follow a consistently-directed sequence of edges that eventually loops back to v again. A DAG is a directed graph that has a topological ordering, a sequence of the vertices such that every edge is directed from earlier to later in the sequence.

A DAG model works differently than a blockchain. A common blockchain requires miners to maintain blocks, but a DAG wouldn't need either proof-of-work or blocks.