36L is Good, 20W is Great and 14H is a little iffy, but I think it would Work.
The Minimum size Enclosure for an Adult Bearded Dragon is 36L x 18W x 18H. And Bearded Dragon's Need Length and Width more than Height. So I would day 36L x 20W x 14H would be a good Buy. You just have to make sure your beardie can Not get to Close to it's Baskinglight.