My husband and I will be in Paris in late September. We have two extra days in which to see Versailles, Chartres and Giverny. Any suggestions on the best way to see these three sites in two days and still have quality viewings?
You should save one of these sights for another trip. Giverny could be combined with a day trip to Rouen (if you are nimble), but these sights do not play nicely with each other.
Sorry to say, since this is probably not what you want to hear, but if you are concerned about quality (as you should be), that's my advice for what it's worth.
If you combine any two, I suggest that it be Versailles and Chartres. Take the RER C to Versailles in the morning. Eat a picnic lunch on the one-hour direct SNCF train ride from Versailles to Chartres. Visit Chartres in the afternoon. Take a SNCF train back to Paris.
To clarify the prior posting slightly.
In order to engage the train you will have to navigate from Versailles Rive Gauche station (where the Chateau is) to Versailles Chantiers station (or vice versa depending upon your direction). Your could either hire a taxis at the station to do this or use Google Maps if you are into a walk (nothing huge but Versailles sometimes confuses people). There are trains nearly every hour. No reservation needed.
Trains between Chartres and Paris are more frequent - especially during peak travel times - some as often as every 15 minutes. These depart from Gare Montparnasse. This trip is a little over an hour.
You can easily combine these two locations - but it will require a bit of an early start. I would recommend that you start in Chartres - because many of the areas of the Marie Antoinette estate at Versailles do not open until noon.
Giverny is much easier as its own trip only because of the direction. It is only a 45 minute train ride - and these are frequent as well - usually hourly. Departures are from Gare Saint Lazare.
In terms of the quality viewing - I can easily spend an entire day at each. But then my attention span is different than most. You may find that with combining Versailles and Chartres it will be difficult to see the gardens at Versailles completely.
Jona's suggestion of starting at Chartres is better than my suggestion of starting at Versailles. Entrance to the Grand Trianon begins at noon.
To get to Giverny, take a train from Gare St-Lazare to Vernon and either a bus or taxi from there to Giverny.