As this answer points out, (and I am sure in the following three years there have been several others) the Falcon Heavy center core is not at all a standard F9. It is specially built to withstand extra loading. It might be capable of launching by itself, but it couldn't be called a F9, and not sure why one would even do that since it's far more valuable as a FH center core.

As far as the two side boosters are concerned, according to this table B1023.2 and B1025.2 have been already used once previously. According to this answer to the question Has SpaceX re-used a first stage twice yet? they are not likely to be used a third time any time soon. However, so far I am not aware of any reason why they couldn't be used a third time, and in the long term F9 re-use multiple/many times is planned.

That last part was originally written as an answer to this question.

The flight has now happened, yay!

As far as the two side boosters are concerned, according to this table B1023.2 and B1025.2 have been already used once previously. According to this answer to the question Has SpaceX re-used a first stage twice yet? they are not likely to be used a third time any time soon. However, so far I am not aware of any reason why they couldn't be used a third time, and in the long term F9 reuse multiple/many times is planned.

The above was originally written as an answer to this question.

As this answer points out, (and I am sure in the following three years there have been several others) the Falcon Heavy center core is not at all a standard F9. It is specially built to withstand extra loading. It might be capable of launching by itself, but it couldn't be called a F9, and not sure why one would even do that since it's far more valuable as a FH center core.