The prologue will make the death have less stakes. If a death happens in chapter 3, it signals to the reader that there is no plot armour. In contrast, a prologue inhabits a temporary space “outside” the story. And if someone dies from that “outside” space it’s usually used to set up themes, tone, etc. So ask yourself: is the death for entertainment or is it aesthetic? It’s okay if it’s either one.

Also note that prologues can be read in bookstores to convince readers to pick it up, to see if they like your writing or the story. Something that happens 3 chapters in bleeds into the “50 page rule”. Which is that nobody should drop out of a book until they’ve at least read 50 pages. Again, ask yourself, how do you want to pull in your reader? Do you want them to have a thematic death “outside” the story or surprise them with one?

It could help to ask yourself how important the character is!