Although it is fair to call all these games a series, they are not necessarily the same system.
They do share some common elements. All of them have a defined starting square and players draw a tile and expand the map as part of their turn. Players have a basic hand size of 3 cards and movement and combat are resolved with a d6.
Zombies!!! (and all the games with Zombies!!! in the title) is a line of games published by Twilight Creations, Inc.. The games are known for their simple play, good representation of theme, and for coming with hundreds of little zombie miniatures. The mechanics are fairly simple and the players collect bullets and hearts to survive. In most cases, the winner is the first person to collect 25 zombies or reach a specific tile.
MidEvil is an evolution of the Zombies!!! design. It features a resource management aspect because the opponents are Skeletons of different colors: white, red, and blue. The skeletons replace the bullet tokens of Zombies!!! and are also used to play or utilize certain cards.
Humans!!! is probably the least similar in system to the original game. Players are the zombies and they attempt to either infect or eat enough humans to achieve a victory point goal. There are variable player powers and victory conditions.