If he's traveling in March, the Three Rivers will be extinct already (unless the travel date is before March 6). I'd be weary of booking travel on the 3R, though, as Amtrak doesn't treat a train very well when it plans to discontinue it.
On New Year's Eve, a freight train derailed, blocking the Capitol Limited's route from DC to Pittsburgh. As a result, Amtrak "terminated" the Three Rivers in Pittsburgh, "vanstituted" the Ohio passengers, and used the equipment to run PGH-CLE-CHI.
A couple of weeks ago, when the Lake Shore Limited derailed a lounge in Indiana and had to leave its lounge and coaches behind, Amtrak cancelled the Three Rivers that night, and used its lounge and coaches to make up the LSL's consist.
So, essentially, if there's any service disruption affecting an eastern LD train, the Three Rivers is invariably going to suffer because of it.
As for the Empire Builder, the best scenery is eastbound, as the westbound tends to travel through Glacier Park after dark. Of course, the tradeoff is that the eastbound train can miss some of the scenery either along the Columbia River (Portland) or the...whatever River (Seattle), because it can get dark shortly after you leave the origin city. Not sure what the daylight situation will be like in March.
Zephyr has good scenery through the Rockies no matter which direction you travel, but if you've already done it, you might want to see something new.
The Sunset isn't a bad train, you just have to consider yourself lucky if you arrive in LA on the same *day* you were scheduled. We'll see how lucky I get in a week and a half.