Zirax tier 3 and above POIs.

I took over Defense Satellite level 3 and I can't find Data Core anywhere, is it in some special container?

I believe it is the POIs with 12 or more guns on it and it has a basement. 12 / 4 is how hard the POI is. The other number has to do with how hard the enemy foot soldiers are.

Levels and tiers are completely different. Level refers to the NPCs contained within, and how much damage they will do and how much it will take to kill them. Tier refers to the type of POI. The Zirax Sector command is a Tier 5 POI, The Ghyst Military Bunker is a tier 4, the Drone base (NOT the temporary drone base) is a Tier 3 (as examples). You will NOT find the Tier of a POI written down anywhere within the scenario, it's knowing what POI is what and how each POI is built up, how many defensive weapons? Does it have a shield? Does it have a basement section? And so on, and so on. Each tier builds on the previous. From Memory, a Tier 3 would have a MINIMUM of 12 turrets, a shield , and laser fences.
Drone Base (T3), Missile Base (T3), Military Bunker (T4), Fusion Power plant (T4),
Sector Command (t5)

(This is from Oct last year, may have been revised, but is still pretty current as far as i am aware)
Tier 1 POI = 1 Core (Most common POIs, 4 turrets, no shield)
Tier 2 POI = 2 Cores (8 turrets, shield, walls, no underground)
Tier 3 POI = 3 Cores (12 turrets, shield+, walls, underground level)
Tier 4 POI = 4 Cores (16 turrets, shield++, walls, multiple underground levels)
Tier 5 POI = 5 Cores (20+ turrets, mega shield, multiple walls, extensive buildings, multi-level dungeon both up and down)

From what I understand there no tiered space stations, you would only find data cores on Zirax OPVs, again tier 3 and above. Imerat-Class Destroyer, Kamirat-Class Battlecarrier, Kythera-Class Battlecruiser, Tovera-Class Dreadnought.