For the third ability, the power and toughness of the creature will change as the number of lands you control changes.

Gaining a level is a normal activated ability. It uses the stack and can be responded to.

You can't activate the first level ability of a Class unless that Class is level 1. Similarly, you can't activate the second level ability of a Class unless that Class is level 2.

If you have more than one Druid Class at level 2 or higher, you can play that many additional lands per turn. For example, with two Druid Classes at level 2, you can play three lands during your turn.

Gaining a level won't remove abilities that a Class had at a previous level.

Each Class has five abilities. The three in the major sections of its text box are class abilities. Class abilities can be static, activated, or triggered abilities. The other two are level abilities, one activated ability to advance the Class to level 2 and another to advance the Class to level 3.

Some Class cards have an effect that increases when more are under your control. For example, if you have multiple Barbarian Class cards, you roll that many additional dice and ignore that many of the lowest rolls.

Each Class starts with only the first of three class abilities. As the first level ability resolves, the Class becomes level 2 and gains the second class ability. As the second level ability resolves, the Class becomes level 3 and gains the third class ability.

You can multiclass or even control multiple Class enchantments of the same class. Each Class permanent tracks its own level separately.

Whenever a land you control enters, each landfall ability of the permanents you control will trigger. You can put them   on the stack in any order. The last ability you put on the stack will be the first one to resolve (As a result, you can have those abilities resolve in the order of your choosing.).

A landfall ability triggers whenever a land you control enters for any reason. It triggers whenever you play a land, as well as whenever a spell or ability puts a land onto the battlefield under your control.

A landfall ability doesn't trigger if a permanent already on the battlefield becomes a land.