The reason sha-3 doesn't appear when you type
sha is not because it contains a hyphen, but because there are 6 tags that contain
sha and that are more popular. Renaming the tag to sha3 would be typographically incorrect (it's “SHA-3”, not “SHA3”) and would not help you.
There is the question of to what extent we want tags for specific hash functions. A majority of questions on this site (unlike Cryptography) involving hashes are about using a cryptographic hash to ensure the integrity of some data and not about a specific hash function. However, there are questions where the specific hash function is relevant, so these tags are sometimes warranted, but not everywhere they're currently used.
There are currently 4 questions tagged sha-3. They're about moving to SHA-3 from SHA-2, so the tag is relevant.
There are currently 102 questions tagged md5 (which is the proper spelling). Some of these are about the known security problems with MD5 and whether they affect a system using them (example); the tag is very much warranted there. Others are about hashes in general (example) and should not have a tag for a specific hash.
sha1 (which should be sha-1) is currently synonymized to sha. This makes some sense, though I suspect a few of these should really be tagged sha-1 as they're about the abandonment of SHA-1.
sha2 (which should be sha-2) is a separate tag, as is sha256 (which should be sha-2-256 or sha-256). As SHA-2 is the “default” hash family these days, most of these questions should really just be tagged hash.
There's even a tag sha with 89 questions. At a glance I only see questions about hashes in general (it just happens that the most popular hashes are of the SHA family), in which case hash should be used instead, or occasionally about a specific member of the family, in which case sha should be replaced by e.g. sha-1.
- Merge sha, sha2 and sha256 into hash. (Moderator action.)
- Remove the sha1, sha2 and sha256 synonyms. (Moderator action.)
- Manually retag questions that are specifically about SHA-1 to use sha-1. (Community action.)
- Retag the questions currently tagged md5 but not specifically about MD5 to hash. (Community action, can be done in parallel with the above.)