What size do I need?
Measure the inside diameter of your portafilter basket. Most home machines use 58.5mm. Breville machines use 53.5mm. Smaller single-spout portafilters typically use 51mm. When in doubt, check your machine's manual or measure the basket with a ruler.
Do they actually make a difference?
Yeah. The screen distributes water more evenly across the puck, which reduces channeling and gives you a more consistent extraction. You'll also notice your shower screen and portafilter stay way cleaner between shots.
How long do they last?
They're made from 316 stainless steel, which is the same grade used in marine and medical equipment. With regular rinsing they'll last years. Most people never need to replace them.
Will it affect my shot pressure?
Slightly, but in a good way. The screen adds a small amount of resistance that helps with water distribution. You might need to adjust your grind slightly finer when you first start using one, but most people dial it in after a shot or two.
Why three in a set?
So you always have one ready. Toss a dirty one in the sink, grab a clean one. No waiting around between shots. It keeps your workflow moving, especially if you're making drinks for more than just yourself.