All of this depends on OP's risk tolerance. There is no such thing as a layover and he is not on a stopover, he is on a connection. A 3-hour connection is 2:15 over the MCT, so DL will see this as a fraud, largely because DL gets tons of these at ATL because tickets to/from hubs are expensive and there are folks looking for 6-7 hours at ATL who book a connection and then "miss" it.

So, maybe his onward segment and any remaining segments will be cancelled and won't have any value if he has a typical discounted ticket and does not cancel prior to departure. In that case, he will need to purchase a walk-up fare and perhaps wind up in a middle seat near the lav if he's lucky.

On the other hand, OP may catch some agent's sympathy and, if he does, he will be rebooked.

On the other hand ATL-LAX flights often overbooked, so next available may well be sometime the next day and OP will be stuck with hotel & meals for the overnight.

So, bottom line is that even if the last 999 people who did this got away with it, OP may be the 1,000th and hit the agent who just got dinged for helping a passenger commit a ticketing fraud and therefore is sensitive to the issue.