I don't think the cause for ERR_FIL_PACK_3 is correct. I don't have any DLCs over 4GB (biggest one is 2.3GB) and I'm having this issue.
My game was working fine, I decided to install the Faster AI Drivers mod, and suddenly I'm getting the error. The ridiculous part is that I'm still getting the error after removing the one file the mod installs.
EDIT: Seems that OpenIV had broken mods\update.rpf somehow. I copied every modded file I had from it to the desktop, re-copied the original one, then dropped all my mods back on it exactly as they were and it suddenly works. GTA is a weird one...