Start off by creating a taskforce with the creature(s) you want to spawn. For the random effect, I've added 3 taskforces (1 fiend, 1 floater, 1 visceroid). Then create a script(s) for each of them. The important part is that the script is never ending (use script action Jump to line #), otherwise the team will cease to exist. Next the teams for each one:
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Note they all share the same Tag, Tib 1 Dead. So let's create that trigger.
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When any of these creatures dies, 3 spawning triggers get enabled. One of them will activate first (spawning one of the creatures), while the other 2 get disabled. This is based on randomization. We need the spawn triggers (3 of them). Here's Spawn 1A defined (just clone this one and adjust it for the other two, namely changing the team that gets spawned):
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When the creature dies, all 3 spawn triggers are enabled and the random delay will make one of them activate first. The activated one then spawns one of the three teams, and all the spawning triggers are disabled (until the spawned creature dies). The value in Random delay defines how long it will take for a replacement creature to appear (+-50% of the value). You can also make it more likely for one of the creatures to appear, e.g. by giving Tib 1A a lower Random delay than Tib 1B and Tib 1C.
This randomization technique can be used for alot of other stuff on maps as well, for example to define whether an ion storm starts or not, where the objective (e.g. an enemy commander) will appear, etc.
With these your map will keep respawning replacements for dead tiberium creatures (with the delay you wish). It will also be quite random since every time you could end up with a different creature. Even more random if you give each spawn trigger a different waypoint. Also this example was for only 1 creature, you should probably create more of them (it can lead to alot of triggers, so name them properly to keep it organized).