I do not know what you will be doing in Frankfurt for 3 days with no vehicle. If you rent a car in Frankfurt you could drive south an hour and visit Heidelberg.
Most car rental companies do not charge extra to drop of a car at a different location so you could see some nice areas driving back to Frankfurt. Rent car BEFORE LEAVING USA. VERY expensive rented in Germany.
I was stationed north of Trier many years ago and have visited a couple times a year since 1999. For good hotels in Trier the Tulip Inn (with car)city bus ends in parking lot, or the Ramada (book thru Marriott web site from pull down menu stating Ramada International)usually have great deals. Both have good breakfasts included in price and can be as low as (single)$45.00 Tulip Inn and $65 Ramada.The Tourist Bureau near the Porta Nigra can help in a pinch. Try www.trier.de and they tel you the deals of the month. You can get a newsletter and order tourist stuff/info in English. I would drive back to Frankfurt along the Mosel along route 50. But your way is probably just as good since the wine
fields are bare this time of year.
If I can provide more info to you write and I will give you my e-mail. Hope you have a great time