What Model Year? Mods?
If I understand, once you rev above 3k for the 1st time, it will stay at 3k and never drop. If so, this tends to rule against an IAC issue being the primary problem.
To me this sounds like:
* Loose, worn, or damaged throttle linkage.
* Vacuum leak.
Of course this could be two problems. IE, IAC and TB linkage. Or the backfire may have dislodged/broken/burst a vacuum line. The backfire could have also damaged the throttle linkage or throttle body.
Start by performing a visual inspection. Look for ANY loose or damaged vacuum lines, PCV lines, Oil dip stick, Oil filler, TB linkage, upper intake, or any other source for a vacuum leak.
Disconnect the electrical connection to the IAC and report what happens. The idle should drop. If not, suspect an IAC problem or vacuum leak.