Three lists, all legal, all exactly 3 DP.
A walker component: [Shield Host](/40k/armies/adeptus-custodes/detachments/shield-host) (2 DP) + [Might of the Moritoi](/40k/armies/adeptus-custodes/detachments/might-of-the-moritoi) (1 DP). A generalist core whose Martial Mastery improves every melee attack made by a model with the Martial Ka'tah ability, which includes your Dreadnoughts, plus walkers that actually arrive. The two rules compound on the same models: the Armour Penetration bullet on the round your Contemptor charges, Rendax Stance for [LETHAL HITS] on the swing, and Interred Expertise re-rolling the 1s underneath it. This is our recommendation for a walker-curious first list.
A walker army: [Solar Spearhead](/40k/armies/adeptus-custodes/detachments/solar-spearhead) (2 DP) + [Silent Hunters](/40k/armies/adeptus-custodes/detachments/silent-hunters) (1 DP). The armour does the work, and the Sisters run the objective actions that tanks structurally cannot, which is the specific weakness of every vehicle list. Silent Hunters carries no Unique Tag, so there is no conflict with ARMOURY.
A two-threat army: [Might of the Moritoi](/40k/armies/adeptus-custodes/detachments/might-of-the-moritoi) (1 DP) + [Tharanatoi Hammerblow](/40k/armies/adeptus-custodes/detachments/tharanatoi-hammerblow) (1 DP) + [Silent Hunters](/40k/armies/adeptus-custodes/detachments/silent-hunters) (1 DP). The only legal three-detachment Custodes army. Walkers advancing up the middle, Allarus arriving behind the enemy, Sisters scoring. Their tags are ARMOURY, LIONS and none, so nothing collides.
There is one other pairing this faction blocks, and it works the same way: Lions of the Emperor and Tharanatoi Hammerblow share the LIONS tag, and both print an identical stratagem called Unleash the Lions. The pattern is worth internalising. In an Adeptus Custodes army, the more perfectly two detachments seem to double the same idea, the more likely a Unique Tag stops you. The tags are drawn around exactly the effects that would stack.