The concept of blockchain 3.0 consists of using distributed ledgers to store any kind of information other than crypto-currency or financial information. The main idea lies in applying the blockchain technology to secure different kinds of applications, as for instance, health care, electronic voting, and IoT. However, in general, these applications require distinct types of blockchains, with diverse characteristics. As a result, each application employs its own blockchain solution that cannot exchange information with other applications. Hence, blockchain interoperability has become an important issue, since it allows not only exchanging data among applications but also to offer the possibility of developing multi-blockchain systems. Using multiple blockchains can improve the scalability of the system, which is a fundamental issue in blockchain technology. Besides, it can also help to organize data into different blockchains, allowing a more efficient access control. Therefore, in this paper we present the main solutions in the literature to enable cross-chain communications, which is the key issue to accomplish blockchain interoperability. We present solutions with different approaches to cope with cross-chain communications. We also investigate the benefits of smart contracts to implement cross-chain communication.