Lightroom HDR - unusable results

There won't be very much to correct it. AI denoise does not work on an HDR merge, because merged images are no longer mosaiced raw. The question is what caused this and how to avoid it, not how to correct it after the damage has been done. Several things come to mind. First of all, what ISO did you use to shoot the brackets with? Secondly, what did the brackets look like? Did you have a properly exposed image, a darker image and a brighter image, or were all three images very dark? And finally, did you use heavy deghosting? Deghosting takes the pixels from one image rather than merging three images on the spots that need to be deghosted. And too heavy deghosting can cover large areas, much larger than really necessary.

In the bracket of images for the merge, where any way way underexposed, or way way overexposed as in complety or nearly blown out?

In the Merge Photo HDR options, did you have Auto Tone checked. And if so, did that result in some over done develop module tone settings perhaps involving contrast including clarity?

Have you tried the denoise tools?