The Last Kids on Earth 3 Books Collection Set by Max Brallier (Books 7-9) (Doomsday Race, Forbidden Fortress, Monster Dimension)

Doomsday Race:
With his zombie-controlling powers growing stronger, Jack Sullivan and his buddies are road-tripping toward the mysterious Tower, where they must once and for all stop Rezzoch the Ancient, Destructor of Worlds, from descending upon our dimension. But their journey is sidetracked when they are swept up by the Mallusc, an enormous centipede monster carrying the world's largest shopping mall on its back.

Forbidden Fortress:
Picking up after Quint and Dirk's Hero Quest, the Last Kids are happily reunited―but quickly faced with a monstrous new mission. Inside an other-dimensional fortress, the evil Thrull, alongside a vile new villain, is carrying out a sinister plan. Jack, Quint, June and Dirk must make their own plans to infiltrate the stronghold before Thrull gets any closer to completing the mysterious Tower, a structure that could ultimately spell doom for this dimension.

Monster Dimension:
The last kids may have escaped the forbidden fortress by the skin of their teeth, but there isn't much to celebrate. Thrull has what he needs to complete the Tower, Rezzoch will most certainly come to Earth, and the world as they know it will cease to exist. Except Jack's Cosmic Hand is now even more powerful than he realized, and it might just be able to stop the inevitable from happening. To get the answers he seeks, he'll need to travel farther than he's ever been. He must go . . . to the monster dimension!