Visiting Frankfurt on a 3-hour layover

No, this isn't feasible. It will probably take you around 30 minutes to get off the plane and through immigration (and probably more if you're not an EU citizen). It's then about a 15-minute train ride into Frankfurt, and 15 minutes back, plus, say, another 10–15 minutes total waiting for the two trains. But you're recommended to be back at the airport two hours before your flight leaves, to make sure you get through security and so on. That leaves you with about minus-10 minutes to spend in Frankfurt.

Even if you decide to push it and not arrive at the airport until an hour before your flight, you still only have 45–50 minutes in Frankfurt, plus the stress of getting back and worrying about missing your flight. It's unlikely to be worth it; just eat something at the airport.

No, this is not a good idea.

To extend on @DavidRicherby's answer. I just did this a couple of months ago, but on a 9 hour layover.

From plane to train took a little over an hour and a half. I arrived at 8:50 in the morning and I know the train I took was right around 10:36.

The lines were long and the place was quite crowded. Not only that but I had to figure out where to go and then figure out the train ticket kiosk and how to buy the ticket (that took 10 minutes after asking for help)

Now, I went to Mainz and not Frankfurt, but the situation is the same.

Getting back took slightly longer because I had just missed the train on the way back. So I had to wait. I'm guessing it was no more than 15 minutes, but it felt longer.

Once I arrived, getting from the train to the plane took slightly over an hour. Again, it was crowded and lines were long.

So, even if you were able to go and grab a snack, you will be running, stressing and it's not worth it.

You have to factor in variables such as traffic, what happens if you miss the train, what if your food comes late (service could be slow), immigration lines could be long.

All in all, I spent about 2.5 hours leaving the airport, and getting to my departure gate (and that was just IN the airport).

Too close risk it on a 3 hour layover.

Simply put it's a bad idea and you'll probably not be able to do or see much even if you decided to go ahead and manage to pull it off.

You have to consider all the time you'll spend leaving the airport, waiting for the train, travelling on the train, then multiply that by 2, because you want to return and get back on your connecting flight. Even considering an unlikely alignment of events would still make the attempt not worthwhile, other than to get an anxiety rush.

I visited Frankfurt on a 6-hour layover, which is double your layover. It was enough to be relaxed about it, but you still don't get to do or experience much, because you need to be mindful of your return journey.

Given the amount of time and stress you'd spend on this plan, I'd rather enjoy a nice meal in the airport itself.

That's not really feasible. A 3-hour layover is much too short, and the airport is FAR from downtown. Public transport is not like hopping in a Zipcar, there's a lot of fooling around and waiting. And, things can go wrong - one mistake or delayed train and you're out an expensive air ticket.

I wouldn't have attempted it before 9/11 on a domestic flight. If all you did was clear immigration, step outside, take a breath, turn right around and queue up for security - you'd have just about enough margin for error.

If you want to do that sort of thing, travel by rail. The depots are usually downtownish. I've had a lovely meal in downtown Denver and explored bookstores, I've walked up from NYC Penn to Times Square, did a nickel tour of one Smithsonian museum or another, and gotten Uno's Pizza in Chicago... All on less than 3 hour layovers. And that's just in the US, which has notoriously awful rail service. Europe is a wonderland by comparison.

Or if you must do it by flight, beg your agent to pad your connections out to about 8 hours.