Is your object flipped over after importing it into RCT3? Well, then this is the tutorial you need. You should be able to understand this tutorial by simply looking at the pictures, the text is just an extra guideline.
First of all, enter the importer, and click Create OVL.

Next, load the file you wish to fix. You can skip this part if it is a new file.

Select the file you wish to load, and click OK.

Click on the model, and press the 'E' key, entering edit mode.

Under the transformation matrix, click edit. A new window should pop up.

In the new window, click Fix Orientation. Notice the words 'Legacy Fix Orientation' on the list to the left. This will also change the numbers on the table below the list.

Click OK. This should exit the window.

Click OK to close the next window.

Save the file.

Click Create OVL.

Click Save. If it was already saved, over-right the old save.

Repeat with all objects that need to be fixed. When finished, click Install.

You may also want to save this, by clicking file -> save.
I hope this tutorial has helped you.