Here's what the Oxford English Dictionary has to say about each:

Terminus:

Etymology: < classical Latin terminus boundary, limit, end, (with capital initial) the god who presided over boundaries and landmarks, in post-classical Latin also fixed date or point in time (frequently from 8th cent. in British sources), mathematical expression (see term n.) < the same Italic base as Oscan teremenniú (plural) boundary stones, teremnattens they have delimited, Umbrian termnas delimited, termnom (accusative singular) boundary stone < an extended form of the Indo-European base of ancient Greek τέρμα end, boundary.

Hermes:

Etymology: < Latin Hermēs, Greek Ἑρμῆς.

With the first listed definition being:

In Greek mythology, a deity, the son of Zeus and Maia, represented as the messenger of the gods, the god of science, commerce, eloquence, and many of the arts of life; commonly figured as a youth, with the caduceus or rod, petasus or brimmed hat, and talaria or winged shoes. Identified by the Romans with Mercury.

So "Hermes" is just a modern Anglicization of the same word in ancient Greek, with the same meaning, i.e., the name of the Greek god.

Now the tricky one. Mercury:

Etymology: < classical Latin Mercurius the god Mercury, cognate with merc- , merx commodity (see market n.): the suffix -urius is perhaps of Etruscan origin. In sense 3d probably after Middle French mercure (c1550 in this sense, 15th cent. in sense 7a).

The Roman deity was probably originally the god of commerce only, but is in classical Latin literature completely identified with the Greek Hermes. In early times in England, Mercury seems to have been identified with the Germanic Woden: compare quot. OE at Maia n., and discussion s.v. Wednesday n. and adv. The transferred application to the planet is found in classical Latin; like the other planetary names (the Sun, Moon, Mars, Jupiter, Venus, and Saturn corresponding to gold, silver, iron, tin, copper, and lead respectively), Mercurius in post-classical Latin came to be used to denote a specific metal (from 12th cent. in British sources). This use probably arose from analogy between the fluidity of the metal at room temperature and the rapid motion held to be characteristic of the classical deity.

A little bit more info is found at the entry for "market". The OED says that the "immediate origin" of the word "market" is "uncertain" because many different languages had adopted some form of the word into their language before English did, but that all these words ultimately come from Latin. So English could have borrowed it from some other language, or else directly from Latin. Either way, from Latin, the etymology is:

< classical Latin mercātus market, fair < mercārī to buy, to trade ( < merc- , merx commodity (compare Oscan amiricum , amiricatud ; origin uncertain)) + -tus , suffix forming verbal nouns.

So, here, the OED says that Mercury ultimately derives from the word merx or merc-, meaning "commodity", which didn't only show up in Latin, but appeared in another early Italic language, Oscan. Which means the word probably has a common ancestor to both, but that origin, says the OED, is "uncertain".

Turning, then, to Michiel de Vaan's 2008 Etymological Dictionary of Latin and the other Italic Languages, he gives the following explanation for the origins of the Latin word merx/merc-.

All derive[] from a stem *merk- also found in Faliscan and Oscan. The god Mercurius was probably the god of exchange. According to WH, the god's name was borrowed from Etruscan; in principle, the same is possible for the stem *merk- altogether.

The "WH" referred to is the 1934-58 edition of the German-language etymology book Lateinisches etymologisches Wörterbuch by Walde and Hoffmann. That book went through several editions - Walde actually died in the 1920s before Hoffmann took the project over. You can find early public domain versions of the book, which have a thorough entry for merx, but it doesn't appear to mention the Etruscans (though I can't read German). It may have been something Hoffman added later.

In summary:

Terminus comes from a Greek word meaning "boundary". The word was borrowed into Latin, and from there, adopted to the god of boundaries and landmarks.

Hermes comes from the Greek word meaning the same thing as it does now - the name of the Greek god of commerce. The ancient Greek was Latinized into classical Latin, and then Anglicized into modern English.

Mercury comes from an unknown origin, possibly Etruscan, or possibly some sort of Italic or proto-Italic language predating Latin and Oscan. The original word meant something like "commodity", and had possibly already been adapted to the name of the god of commerce in Etruscan, when these variations of the word were borrowed into Latin. Unlike the other two words, there's no indication the word originally derives from Greek.

No problem! But yes, I imagine a lot of Wiktionary's entries are copied from there.