After launching our game, right after we load our parksave when we first try to make our way about our park we occasionally find that the keyboard is not responding in our game. This problem occasionally occurs whether we play RCT3 in fullscreen or in windowed mode.

This simply means that RCT3 doesn't perceive itself as the active window on display at the time. It is a temporary problem that is easily fixed with any of the following methods:

Use The WIN Key

After pressing the Windows key, left mouse click on the RCT3 display. After doing this you will be able to use your keyboard as regular.

Use ALT + TAB

In the popup that displays select another icon which will make that application the active window on your computer. Then go down to your Taskbar and left mouse click on the RCT3 icon (or left click somewhere on the RCT3 display if it is in windowed mode). After you've done this RCT3 will become the active window on display and you’ll find your keyboard now works as it should.

Use CTRL + SHIFT + ESC

If there are no folders opened and no other applications running you might want to use CTRL + SHIFT + ESC to bring up Task Manager, after which you may left mouse click on the RCT3 icon in the Taskbar (or left click on the RCT3 display if it is in windowed mode). This will again make RCT3 the active window on your display after which your keyboard will be responsive.

With the inexplicable now resolved, this annoyance will end up a minor problem.