Li Shenron
Legend
I sounds like your problem is less the actual power and more that the only current Minor Invocations are a direct damage invocation(Eldritch Blast) and a movement invocation(Shadow Veil).
Yes and no... If there were more lesser invocations AND eldritch blast wasn't automatically known, then it would be much better already, but still 3d6 sounds just like every Warlock would anyway pick it up (in fact this may be because it's automatic... because as it is they know they would have it anyway).