The 'Open Image' button is a global option which will be present in the Button Bar for all images (or none at all).
However, you could perhaps achieve something similar by using one of the following suggestions.
You could set imageClickMode="OPEN_URL" and set a corresponding linkURL (which would be opened when the user clicks the main image) for each of your three images. For the other images in the gallery, you could leave their linkURLs empty (and SimpleViewer would just open the image itself in a new window when the user click on the image).
SimpleViewer Support Team