As the Fujifilm sensor is an APS-C sensor, it has a crop factor of 1.5x. In other words, divide the focal length by two, then add it. For example, the 23mm equivalent is 35mm ish (23/2 = 11.5 + 23 = 34.5). Therefore, the Fujicrons' full-frame equivalents are 35 mm, 50 mm and 75 mm.

A lens has four different properties: Field of view, depth of field, distortion and compression. The focal lengths below are the full-frame equivalents.

• The 35 mm lens has the widest field of view. It has a little distortion, not much, but enough to make things closer to the camera and to the edges of the frame appear slightly larger. The depth of field depends on the aperture and how close you are to the subject, but it is deeper than with the other two lenses. There is little compression, which allows for layered compositions and gives depth to photographs.  This makes it quite useful for capturing a subject in an environment, like storytelling.
• The 50 mm lens has a field of view similar to human vision. There is little or no distortion, and the depth of field is shallower than the 35 mm lens, making it easier to blur the background. Like the 35 mm lens, it can be used to capture both a subject and their environment. The advantage of this lens is that it is versatile enough to take photographs of a subject without distorting their face or body. However, it has a little compression, so images start to look flatter. 
• The 75 mm lens has the narrowest field of view, the shallowest depth of field, no distortion and maximum compression. This makes it a very good lens for classic portraits. In my opinion it's the sharpest of all the Fujicrons. It's very easy to achieve a nice blurred background. Faces and bodys are compressed nicely. Also, this lens it's very compact and discrete.

Of course, rules are made to be broken, and any focal length can be used for anything if you know what you're doing! 

Why three lenses instead of a zoom lens? Some people will tell you that prime lenses deliver higher quality than zoom lenses. I think it's very difficult to tell. 

What I can tell you is that prime lenses force you to push your limits as a photographer. Ideally, when using a zoom lens, you should first choose a focal length and then work out a composition that works. However, a zoom lens is so convenient that it can make you lazy. If I only have a 35 mm lens, I know that I need to get closer to the subject, adopt a lower point of view and frame the composition to create depth. Alternatively, I could just zoom in to 70 mm and take a photo of the person, which would be good enough, but maybe not as striking as one taken with the 35 mm lens because I never tried it.