Probably no need to worry about adding just one when it's not a pre-pay like CRT.
You might have to wait longer for a table than you'd like, but I'd show up and simply say you've 1 more. If you were 4 wanting to add a 5th person, that would require a 6-top--a problem at such a busy restaurant.
The two of us are often seated at 4-tops, so I highly doubt 3 instead of 2 will be an issue other than a lengthier wait.
For places that busy, in future I'd book for 4 instead of trying 3 at all. The Disney system doesn't like odd numbers.