I ordered the essential kit and ordered another generator. Is this enough to start or is there anything missing I did not buy?

The kit comes with everything you need for the two generators.
The 3rd generator you have can make use of the items in the kit that are not in use.
If you are using remote mode for the two generators from the kit, you can use a contact connection on the 3rd generator.
What you are missing to make the 3rd generator 100% complete as if you bought a 3 gen kit, is the following:

Another boost 2.0
Another white (BN) remote.
One more spare communications board.

You could easily wire up your 3rd remote to the generator directly to output 1. If you used both white (BN) remotes, you will have a black (MN) remote left for example.

So you can make use of the 3rd generator, but to set it up with the same configuration as the other two from a remote perspective, you will need a boost 2.0 and another white (BN) remote.

The black (MN) remote is for use on a limited basis and is not suitable for long term programs for most people.

If on the other hand you ordered the 3rd generator to drive the Spooky Central, then you do not need to get anything else provided you plan to use the generator only with the SC.