What's Specific to Each Product
The rules above cover all three. These are the few things that differ.
Roller banners are your main, most stable display, the one you take to event after event. Because the graphic retracts into its cassette and can be reprinted separately, it is worth designing as your primary, most polished piece. If you are only doing one indoor display well, make it this.
X banners are lighter and cheaper, and people usually own several. That makes them ideal for a set: matching X banners marking the edges of a stand, or several carrying different single messages. Design them as a coordinated group rather than as one-offs, since their strength is in numbers.
Greeter stands are the one that differs most. The design wraps around the front and both visible sides rather than sitting on one flat face, so your artwork has to work across the front and the returns, not just as a single panel. Keep the strongest branding on the front where people face it, and carry the colour and logo round the sides so it still reads when the stand is seen at an angle. Remember too that things sit on the top surface, a tablet, brochures, a card reader, so the usable branded area is the front face rather than the top.