If you look at the difference in points between Dortmund and Leipzig, Stuttgart should be safe and they will most likely remain in the UCL zone until the end of the season. However, Stuttgart's struggle is not easy because they still have to face strong teams such as Leverkusen, Dortmund and Bayern Munich in the remaining matches. If he makes a lot of mistakes, it is not impossible that his position in third place will be shifted by Dortmund and Leipzig.
I wouldn't want to compare Dortmund And Leipzig because both this season has exhibited a very good level of inconsistency such that I don't know which amongst them both is even worse so I would say they are both on almost the same performance but then even in that Dortmund still stand out much more than Leipzig would, checking the team depth and strength you will agree with me that Dortmund is ahead of Leipzig.
Stuttgart earlier in the beginning of the season, they did appeared like they were really good to be very impressive this season but then they couldn't survive the pressure again few . months later and they started loosing their form and they could no longer match up with the competition in the league but then they have been able I to maintain a relatively fair position such that they are sure of securing a spot for the next UCL which I consider still a fair performance on their side. I'm still optimistic about Stuttgart been able to maintain their position now till the end of the season.
Dortmund is the team currently at threat by Leipzig for their position in the table, as they are both just a point apart which could mean that a mistake from a single game can get them both reshuffled especially Dortmund if they loose a single game they are loosing their position except Leipzig does same too them they can be relatively safe loosing a game and a draw too wouldn't Doo them much good because they difference still remains too narrow.