Is it possible to entangle more than 2 qubits? If so, is it useful?
Ok, entanglement just clicked for me and totally makes sense.

Yes it is completely possible to entangle more than two qubits!

Entanglement is a fancy word with a really simple meaning.

In quantum computing, you can modify the probabilities of values being read.

If you have two qubits, normally there is some probability for each of the possible states: reading a 0 for each qubit, reading a 1 for each qubit, reading a 0 for qubit a and a 1 for qubit b, and reading a 1 for qubit a and a 0 for qubit b.

When you modify probabilities such that any of these probabilities drop to zero, it limits the possibilities of what can happen when you measure the qubits.

Most specifically, in the two qubit case, you could eliminate the situations where they disagree (states $|01\rangle$ and $|10\rangle$), and make it a 50/50 chance between the states of them both being on, or both being off.

Now, they have NO CHANCE of disagreeing when you read their values.

Who knows how reality "implements" this situation, since distance doesn't matter, but there you go... it's pretty darn simple.

A very simple 3 qubit entanglement would be this: $|000\rangle$, or this: $1/\sqrt{2}(|000\rangle + |111\rangle)$