Time in Writing out of chronicle order

I'd say both options are good.

A sudden jump in chronology in an otherwise linear narrative is likely to confuse the reader and it's how much of a helping hand you want to give them with figuring out what's going on. In either case they'll eventually get there, but likely a bit later without you pointing it out.
I'd say that if you're trying to give answers and be clear with the facts then put in a 3 Hours Ago, and if you want to be more mysterious/uncertain then to leave it for them to figure themselves or maybe even put in some dialogue later into the scene that points out where we are in the narrative.

You can make reference to an event for both chapters. In the first they will be talking about it having happened and in the second it is happening or will happen in the future. You can also reference something that has a known order. Something like- chapter 1 Bill and Ted talking while watching the ninth inning of a baseball game, then in chapter 2 Marty and Doc Brown are talking while watching the first inning of the same game. Create some type of link that establishes a chronology the reader can clearly connect the event and timeframe.

If you jump back in time three hours, I'd prefer to have the "three hours ago" or "three hours before" or similar. I've read plenty of novels where jumps in time are signposted. Not always the "three hours ago" kind of one, can just be a date or time stamp, e.g. "2 Jan 6:15" or "September 1954" or something. Novels that jump around in time are harder to follow so the signposting helps you to keep track. I'd find it bizarre and confusing if a story suddenly jumped back in time three hours without telling me.