To preview how your page or gallery will appear when linked via Twitter card, you may use Twitter's card preview tool.
Follow the link above and login using your Twitter account to access the tool. Once there, paste your page or gallery link into the Media URL field and press the 'Preview Card' button to preview the card.
If your thumbnail does not show, please keep in mind Twitter's rules for thumbnail images:
Images larger than 120x120px will be resized and cropped square based on longest dimension. Images smaller than 60x60px will not be shown.
This should not be a problem for CE3 pages or galleries, however, as both Facebook and Twitter implementations reference large rendition image files.
It is unnecessary to be familiar with Twitter's card API, but I include the link for reference.