Secure Hash Algorithm (SHA) is a hashing algorithm used in secured connections to provide the integrity and authenticity of a message to the receiver. SHA algorithm is the default hash algorithm set in SSL/TLS certificates. SHA-2 is a set of hash functions including SHA-224, SHA-256, SHA-384, SHA-512, SHA-512/224 and SHA-512/256. The most common hash function used is SHA-256. Generally speaking, SHA-2 = SHA-256 and that is the minimum requirement.
Secure Hash Algorithm 3 (SHA-3) is the latest member of the Secure Hash Algorithm family of standards, released by NIST on August 5, 2015. A wider implementation for SHA-3 is probably many years away.
