Two 1 DP detachments cost 2 DP, and a Strike Force budget is 3. That leftover point is not wasted, because there is exactly one thing that fits it.
Add [Heroes of the Uprising](/40k/armies/genestealer-cults/detachments/heroes-of-the-uprising) (1 DP) and you reach exactly 3 DP with the faction's only three-detachment army.
It is legal on both counts. Heroes of the Uprising carries no Unique Tag at all, so it collides with nothing, and the other two carry different tags from each other. No two of the three share anything.
It is also a genuinely coherent list rather than an arithmetic curiosity. Heroes of the Uprising gives the Kelermorph, Locus, Reductus Saboteur and Sanctus re-rolls of 1 to hit and to wound with no condition attached, which is exactly the profile a list like this is missing: something that reliably removes a specific model. And its Living Up to Legend stratagem un-shocks every visible friendly Genestealer Cults unit within 12 inches, which in a horde army built on holding objectives is worth more than the assassination that triggers it, because a battle-shocked unit has Objective Control 0.
Why is it the only three-detachment army? Because the faction has exactly three 1 DP detachments, and any three-detachment shape that included a 2 DP detachment would cost at least 4 DP. Those three are Heroes of the Uprising, Purestrain Broodswarm and Xenocult Masses, and no two of them share a tag, so the single available combination is also a legal one.
The army guide sets out the full arithmetic: eighteen legal multi-detachment armies at 3 DP, of which exactly one uses three detachments.