For Step 3 in particular, just passing is enough and it doesn't matter..whether you are American or foreign graduate. They all look for passing Step 3 on your CV before they decide considering your application. You have to know that many States Medical Boards require having a license to join your fellowship. Step 3 is an essential part of the licensing process. Therefore, Step 3 requirement is not always related to the program itself but also related to the State Medical board governing the State where the program is located. NY State, for example, does not require a license while you are in training and therefore you might see fellows without Step 3. This is not true in the vast majority of the rest of the States.
From my past experience in applying for fellowships, the most two important factors -besides Step 3 - were (1) the name of your residency program, and (2) LoR. The most reputable fellowships won't take residents of weak or average programs...Fellowships in University hospitals are interested in residents of university programs, in contrast to community-based hospital programs. Letters of recommendation are also not less critical than your residency name.
This is just a general impression I had during my fellowship applications and I was even told these things by fellowship directors during my interviews. Experiences between people vary.
Know that if you don't have Step 3 in hand at the time of your application, all other factors won't be of value. After passing Step 3, other factors such as scholarly activities, LoR, residency name differentiate between applicants- Step 3 score was not among them (at least in my own individual experience). Also know that not passing Step 3 will ruin your entire career: won't be able to get license to practice, to sit for your specialty board exams, to get a fellowship (still possible but very tough) and will render all the years spent in training in vain. Therefore, if you pass with 75, you own the world!!. if you fail with one point (74), you will have a gloomy future!!!. i.e. one point in this exam might take you into either way!!!! In other words, Step 1 and 2 get you into the residency and Step 3 controls the rest!!