Just to confuse you even further, Hills are bumping up their juice with 72 hole threesomes!
Kgar,
Books offering 3balls will normally have Day1 etc written with the bet type to make sure (technically it's round 1 rather than day 1 because if it gets delayed etc, it carries over).
In case of tie in a 3ball bet, your return is divided by the number of players that tie (halved in the case of two players....)
Matchups can either be for 18 holes (one day) or 72 holes (standard event length other than last week). Always be aware of which one you are dealing with.