What format are most Hearthstone tournaments using?

I'm getting ready to enter my first Hearthstone tournament. This particular one is using a 3 deck format. But it seems a lot are going to a 4 hero - ban 1 format. What's the most common format especially if you get into the higher level tournaments? I'm considering crafting cards which I haven't done yet (have about 1450 dust at this point), and for this particular tournament at least it seems if I can craft class specific cards to make say a Handlock and a Zoolock deck or two different types of Mages like Secret Mage vs Frost Mage, it would be more efficient use of dust.

Do most hearthstone tournaments run a format that requires competitors to use multiple classes? Is this how Blizzcon and the other "majors" work?

There are several different popular tournament formats and each tournament could run any format it wishes. Most large tournaments are played using the Swiss tournament format, and most small (16 or less competitors) use a Single-Elimination or Double-Elimination tournament format. The "Last Hero Standing" match format was the format used most in "televised" tournaments before 2015 including BlizzCon 2014 and the tournaments leading up to it. In 2015, The "Conquest" match format became the more widely used format used most in "televised" tournaments. Both formats have a standard variation and a banning variation.

Last Hero Standing

• Each player brings 3 decks using 3 unique classes.
• The match is played as a best of 5 games (first player to win 3 games wins the match)
• In the first game of the match, each player secretly chooses which of their decks to play.
• In each subsequent game of the match, the winning player of the previous game must use the same deck. The losing player may pick from any of his or her remaining decks.
• Once a player loses with one of their 3 decks, that deck is considered eliminated and can not be played again in that match.
• With the above rules, a player wins in Last Hero Standing by defeating each of their opponent's decks.

Conquest

• Each player brings 3 decks using 3 unique classes.
• The match is played as a best of 5 games (first player to win 3 games wins the match)
• In each game of the match, each player secretly chooses which of their decks to play.
• Once a player wins with one of their 3 decks, that deck is considered eliminated and can not be played again in that match.
• With the above rules, a player wins in Conquest by winning with each of their decks.

Banning variations

• Each player brings 4 decks using 4 unique classes.
• Before the first game, each player secretly chooses one of the 4 classes used by their opponent to ban their opponent from using.
• Players then reveal their bans. A deck using the banned class is considered eliminated and can not be played in that match.
• Standard Last Hero Standing or Conquest rules then resume now that each player has 3 decks using 3 unique classes.