Germany and the Netherlands 3 weeks

It's the reverse to what you're worrying about - domestic trains are covered by railpasses with no problem but international high-speed trains may have additional fees.

Generally nowadays rail passes are useful for spontaneous travel or if you are doing a huge number of rail journeys, but if you are only making a few city-to-city journeys and are willing to commit to travel at a specific time, it is usually cheaper to buy online advance tickets for your individual journeys. For Germany or international journeys to or from Germany use www.bahn.com (tickets go on sale 92 days before travel date). For journeys between Belgium and the Netherlands try www.thalys.com. There are no advance purchase discounts for domestic rail services in Belgium or the Netherlands, so buy tickets as you go.

We can't offer specific advice without knowing more about the individual journeys you will take.

It's not only a question of "what kind of rail pass...?" Whether a rail pass is what you need at all really depends on the journeys you plan to make within each country - not just the number of journeys, but the LENGTH of each one. A 4-day rail pass can compete very favorably cost-wise with 4 days of travel on individual tickets if the journeys are long ones. The DB Saver Fares Philip discusses for, say, Hamburg - Berlin - Munich - Stuttgart - Cologne are not going to save you much if anything at all over a 4-day pass - these are long journeys and expensive to buy one by one, even at a heavy discount. Railpasses start to lose their luster when you compare them to saver fares for shorter journeys (Frankfurt - Würzburg, for example, only €19 pre-purchased well in advance.)

But if you're making, say, 4 different trips within Bavaria, a rail pass is overkill. It might be much cheaper - and offer nearly as much flexibility - if you just get 4 local day passes for local trains (like the day Bayern ticket) every day than to pay for 4 days of rail pass travel. Or maybe 3 day passes and one Saver Fare for a longer trip (like Garmisch - Bamberg, for example, so you don't have to ride the local trains such a long distance.)

Note that the German rail pass is valid between Brussels and Germany on some journeys - so if you end up buying a German rail pass, you might want to find yourself in Brussels just before crossing into Germany.

DB offers saver fares between the Netherlands and German cities, should you want to enter Germany from there instead - I would check into the saver fares for sure for that cross-border trip as an alternative to a multi-country rail pass. You will find p2p fares in the Netherlands and Belgium fairly reasonable.

The rail pass offers a certain level of flexibility you can't get with the Saver Fares (which specify the exact trains you must take.) This is often worth paying a little extra for. Miss a saver fare train, and you forfeit the ticket. With a rail pass you can go where you want, when you want. If you planned to travel from Cologne to Mainz by rail pass, but then once in Mainz you decide to do an additional round-trip journey that same day to Wiesbaden, you just hop on the train. With a saver fare, you'd need another ticket.

Thanks for the feed back I am planning on going from Frankfurt to gottingen then to Belgium Amsterdam and back to Frankfurt to fly home. I'm thinking of buying the train tickets just before I travel and not buying a railpass I don't think it would be worth it for me.

" I'm thinking of buying the train tickets just before I travel..."
That's the expensive way to do it. 1.) FRA - Göttingen will cost €50 - 60. 2.) Göttingen - Brussels will cost €130 - 160. 3.) A'dam - FRA will cost €80 - 110. Not sure what you'll pay for Brussels - A'dam. If you want to buy at the last minute, then get a 3-day German rail pass for €189 at Frankfurt's main station or at the airport station. It will cover trips one and 2 and trip 3 from the Dutch border to FRA. Then you'd also buy Brussels - A'dam and A'dam to the German border. Check the Belgian and Dutch train sites for prices.

http://www.belgianrail.be/en/Default.aspx
http://www.ns.nl/en/travellers/home

If you buy asap instead, you may be able to pay less for trips 1-3 with saver fares at DB, but at the time of purchase you will have to lock in your train travel to specific trains.

Note that the Dutch rail site does not sell tickets unless you have a Dutch-issued credit card. You can buy domestic Dutch tickets in advance from the Belgian site, but there's no reason to as there is no advance discount for domestic trains in Belgium or the Netherlands, and advance discount international fares can be bought from the German site or the special Thalys site.