Let's wait for the whole story and possible SDR before we assume the slide failed. Speaking strictly as a pilot and nothing more, with a reported fuel leak I would not allow over wing slides on either side to make sure nobody slides in to a puddle of Jet A. The 3 of 4 may be that they deployed 3 of the remaining 4 usable slides as one was deem ed a no-go due to smoke or fire conditions.

That being said, one of the slides may have failed to deploy also.