3 days in Paris

I take it this is your first visit? You have to decide what interests you most. The only sight I know of where an advance ticket helps is the Eiffel Tower, others can tell you more about it than I can. If you want to see a bunch of museums and other sights, the Paris Museum Pass is a good value for the energetic. There's no reason to buy it in advance, buy it at the first museum you visit (preferably a smaller one without too long a line). Besides saving money, you avoid all the lines except security.

Also consider one of the hop-on-hop-off buses to give you an overview and get you to some of the places you want to see. The Batobus is a river version of this, or take one of the narrated cruises on the Seine.

In three days (minus your train time from Amsterdam) you'll just scratch the surface of this wonderful city. Don't try to see everything, assume that you'll return.

Get on line to book the Eiffel Tower the moment the tickets go on sale -- I believe it is 3 mos in advance but check and do it at 1 am your time or whatever. The tickets often are bought up within the first hour of availability. Other than that, you don't need advance tickets. You might want to get a museum pass on arrival if you plan to hit many of the spots covered. At the d'Orsay which is a must see museum for most, it allows you to use a separate line which is often non-existant and even on crowded days runs about 15 minutes; the main line can take an hour or two.

look at a guidebook and choose 4 or 5 things you really want to do in Paris and build the rest of your time around that. e.g. visit Sacre Couer and then do a self guided walking tour of Montmartre, or visit Notre Dame and explore the islands and perhaps a walk into the Latin Quarter and up to Luxembourg Gardens or do the Louvre and have lunch in the Tuilleries.