You might try simply hiding the other images within the group on the page so they aren't visible to the viewer:
Code:
<a href="images/image-1.jpg" rel="lightbox[roadtrip]"><img src="1.gif" /></a>
<a href="images/image-2.jpg" rel="lightbox[roadtrip]" style="display:none"><img src="2.gif" /></a>
<a href="images/image-3.jpg" rel="lightbox[roadtrip]" style="display:none"><img src="3.gif" /></a>
The script should still see the hidden images as part of the group and cycle through them once you click on the remaining image.
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