It is possible (indeed, rather likely) that more than two units can become involved in the same close combat.
A multiple combat is a fight that involves more than one unit on either (or even both!) sides. Some examples of multiple combats are shown in the diagrams below. Unless otherwise stated, all the rules for a one-on-one close combat also apply to a combat with multiple units on each side. Bear in mind, however, that multiple combats often create situations not entirely covered by the rules. I've included a commentary for the most common occurrences here, but remember: if in doubt discuss the matter with your opponent before any dice are rolled.