Tire size goes like this:
205 = the tire casing width, in mm
55 = the aspect ratio or profile -- its a ratio of tire width (from the first number) used to express the casing height; in this case = 205mm*.55 = 112.75mm. This is the distance from the outisde diameter of the wheel to the outside diameter of the tire. In an overall side profile, think of it as the height of one of the sidewalls (where two sidewalls plus the wheel make up the total diameter).
R = radial belted. This is the 'type' of tire. Nearly every you can buy today is a radial. Long story short, older tires used to be "bias-belted", but weren't nearly as good. On high performance tires you will often see ZR
15 = the diameter of the wheel on which the tire will fit in inches.
All three tires listed by the OP are the same width, and will all fit a 15 inch wheel, the sidewall height, and overall diameter will be slightly different thought.
To answer the other question as to whether or not they will all fit on the 6.5" wide rim: probably....... but its always best to check with the tire manufacturer. Every tire is designed to fit on a small range of wheel widths. For example, the tires on my Mustang are 275/40/17's and will fit on anything from an 8" to 10" wide wheel. I have them on a 9" wide x 17" diameter wheel.