These countries (Surinam and Guyana, French Guiana is not a country) are better connected to the Caribbean than they are to South America. Therefore it's really hard to get there from South America.
In my opinion, there's nothing worth seeing in Guyana (Georgetown) and it's very, very dangerous and violent. Paramaribo, in Suriname is worth seeing (but only for just a few days, and I wouldn't consider going to the rest of Suriname as it's also mostly dangerous). You have 2 options to get there: either fly from Brazil or the Caribbean with Suriname Airways, or if you're planning to go to French Guiana first, you can drive up to Saint-Laurent-du-Maroni (in the border with Suriname) and then cross the river to Suriname and get a "taxi" to drive you to Paramaribo.
There are only two ways to go to French Guiana from South America without leaving the continent:
1. Take a plane with either Tam or Gol to Belem or Macapá in northern Brazil (note: it will be 3 flights from Buenos Aires to Belem, and 4 to Macapá). That's as far as Tam or Gol will get you. In Belem or Macapá, you can take a Suriname Airways flight to Cayenne, which is roughly 1 hour long, flies twice a week (so do your planning well) and VERY expensive (it will cost you just as much as the 4 flights to Macapá).
2. The other way of getting there is, again, flying to Belem from Buenos Aires with either Gol or Tam. In Belem, you can take a bus that will drive you up to Oiapoque, at the border with French Guiana. You should know that it will be a crazy, 14-hour-long ride through the jungle (on a rocky road), but it will only cost you around 40 reais. In Oiapoque, you can easily cross the river to French Guiana, to Saint-Georges-de-l'Oyapock.
As I mentioned above, French Guiana is not a country, it is a region of France and the European Union, and administratively it's absolutely the same as Paris, Dordogne, or any other region of France (much like Hawaii is just another state of the US). So if you're only interested in visiting these 3 countries to say that you've visited every country in South America, you can cross this one off your list. There are only 12 countries in South America.
However, it is the most interesting one of all 3. It's basically the most developed area in South America, and probably the richest in per-capita income, as they have the Euro and people there make a lot, a lot of money (salaries are in average 40% higher than in Paris). Of course they have a lot of illegal immigration issues (around 20% of the population is Haitian), but they manage to control that quite well. And, of course, you have the European Space Center and if you're lucky you'll get to see a rocket launch, which is a huge event every month.
Overall, all 3 of them are very interesting countries if you're interested in anthropology as the mix of races that you will see there is nothing like what you'll see anywhere else in South America or the Caribbean.
Also, don't expect anyone to speak Spanish in any of these 3 countries at all. They only speak French and créole in French Guiana, Dutch in Suriname and an African language that is sometimes informally referred to as taki-taki, and English (nothing similar to American or British English of course) and an English-based créole in Guyana.
Good luck!
