This type of door effect is a bit complex. It requires more math than all of the other doors, but it's a pretty simple concept once you understand how it works. It opens three different doors, one at a time, limiting the amount of multiple routes you can take at once.
Create a sector with three other sectors attached to it. Seperate the three other sectors with any type of door you want (I'm using Ceiling Doors in this example).
Place a MULTISWITCH sprite (#146) somewhere in your level, and tag it [ActivationSound,Channel]
.
Place two Activators in each door sector. Now this is a little complicated to explain, so I'll use a quick table to show you how to tag the activators:
Door 1---> Activator#1[0,Channel]
Activator#2[0,Channel+1]
Door 2---> Activator#1[0,Channel+1]
Activator#2[0,Channel+2]
Door 3---> Activator#1[0,Channel+2]
Activator#2[0,Channel+3]
Finish creating your doors if you have to.
Now your three doors will open incrementally in the game!