What are the 3 most significant literary devices used in the short story "A Good Man is Hard to Find" by Flannery O'Connor? What are the effects of the techniques (on one or more aspects of the story like theme or overall message)?

Three literary devices featured in "A Good Man is Hard to Find" are foreshadowing, symbolism, and irony.
The violent ending of the story is foreshadowed by the grandmother's insistence that she dress well just in case "anyone seeing her dead on the highway would know at once that she was a lady." This is also a great way of setting up the grandmother's central problem: her superficiality and short-sightedness, which is redeemed when she sees the lowly Misfit as a fellow human being.
The biggest irony of the story is that the grandmother insists they go to Tennessee so they can avoid the Misfit — only for the whole family to run into the Misfit on the way there. This irony is what brings about the grandmother's redemption as well: she tries to avoid her spiritual awakening, only for it to find her.
One of the major symbols in the story is the car the Misfit and his cronies drive, which is described as resembling a hearse. When the family encounter this car, they are almost literally encountering death itself. Once again, in trying to avoid death (the Misfit) by going to Tennessee, the grandmother has instead run into it. Given O'Connor's strong Christian beliefs, the Scripture line about those who seek to preserve their lives losing it comes to mind.