The evils within the dungeon have been defeated. However, with one last villainous act, they grasped for revenge by collapsing the dungeon around the heroes. There's no time to lose; the heroes must move quickly if they have any hope of surviving.
This scenario features a Round Limit within each region. Once the listed number of rounds have passed, the region is collapsed, instantly killing any PCs left inside and permanently blocking any exits back in. Because this is life or death, a PC may spend 1 Karma to decrease the current round count by 1. If you don't want players to be able to just Karma their way out of danger with a stockpile of it, you could limit this to only one or two total uses, or increase the Karma cost by 1 for each use after the first.
The PCs are forced to escape through a separate wing of the dungeon than the one they entered as otherwise many of the Obstacles would have already been cleared. If that's not possible, you could have the self-destruct sequence also put these new Obstacles in the path of anyone trying to escape.
This is a high-octane scenario; rounds probably take only a minute or so to complete, so keep that in mind when considering the distance the PCs are traveling.
(start) A > B - C - D (goal)
|(Region A) From the Depths|
|All the previous exits have already been sealed by collapsing debris. You see one passage out, but the staircase is long collapsed. You'll have to scurry up the rubble in order to reach safety.|
|Up||B||Rocky Wall (Height 2)|
|(Region B) Strange Device|
|This passage looks to have been a dead-end, but there's no other way to go. Looking about, you see a strange device on the far wall. Maybe there's still hope.|
|Down||A||Rocky Wall (Height 2)|
|(Region C) Defense Chambers|
|This area looks built to withstand an invasion, with barricades and elevated areas for guards to attack intruders. The entrance on the east is sealed with a pair of heavy security doors.|
|Key (instantly defeats Security Door)||Hidden: 1d10|
|(Region D) Collapsing Tunnel|
|The destruction is accelerating. This tunnel has mostly collapsed into murky waters swirling with debris. But, you have a feeling you're almost out of here…|
|This region is submerged and requires swimming. Flame-based light sources do not function.|
Make this a safer scenario by slowing the collapse, eliminating the chase nature. Instead of outright killing PCs when the listed Round Limit is reached, upgrade the Crumbling Infrastructure to Falling Rocks at that point. If you do this, decrease every Round Limit by 1 and swap the Falling Rocks in D for Crumbling Infrastructure (at least until the Round Limit is reached). PCs shouldn't be able to spend Karma to reset the Round Limit.
With three players, you might make the key in C a little easier to find. You may also choose to eliminate the Fortunes in A and B, which could throw the players off and lead to them assuming there's just another one of those in C (especially if there's a Delver in the party). Groups without really good lockpicking will have little chance of getting through the Security Door without finding the key.
The combination of swimming and darkness penalties in D might be overwhelming for some PCs, making it extremely difficult for them to defeat the Murky Pool in time. If you expect this outcome (because your players lack access to Aquatic and have no other sort of light sources), you could relax the restriction on the light by making the region not be fully submerged. PCs must still swim, but they can hold a lantern or torch over the water while doing so to light their way.