Think of the 3-way as the surgical option. One side closed, three open. The air carries on doing what it did everywhere except the one direction causing the trouble.
This is usually the right first thing to try when a single desk, chair or bed has a draft problem and the rest of the room is fine. You are not closing the diffuser, you are not turning it into a jet, you are taking one direction out of play.
Standard body: 24 by 24 inches, 2.75 inches deep, polystyrene, held on magnets against a compatible steel grid or diffuser face.

What does a 3-way diffuser cover do?

It closes one side of the discharge and leaves the other three open. The diffuser passes the same air and distributes it much as it always did, minus one direction. Because the volume now leaves through three openings rather than four, each one carries a little more than before, but the difference is small. Of all six configurations this is the one that changes the room least while still solving a specific complaint.

When is 3-way the right choice?

When exactly one direction is the problem. A desk on one side of the diffuser and open floor on the other three. A bed against one wall. A reception position where staff sit on one flank and the public circulate on the others. If more than one side is causing trouble you want the 2-way or the 1-way, which close two and three sides respectively. If nothing about the direction helps and the air simply should not be there, that is the solid cover. The useful question is how many directions are actually a problem, and the honest answer is usually one.

Why start with the 3-way before anything more drastic?

Because it is the least disruptive change that can still fix the complaint, and comfort problems are rarely as big as they feel. Closing one side often does the whole job. Jumping straight to a solid cover removes an outlet from the system and can push the problem to a neighbouring diffuser, while a 1-way concentrates the full discharge into a single hard stream. Trying the gentle option first costs the same $89.99 as every other configuration in the family and frequently ends the conversation. If it does not, you have learned something about the outlet and can step up to a 2-way knowing why.

Will it fit your diffuser?

Three checks, same as the rest of the family. The diffuser should be a nominal 24-inch square, the standard ceiling grid module. You need 2.75 inches of clearance below the ceiling line. And the attachment surface has to be steel, because the cover holds on magnets alone. Aluminium and painted plastic will not take it, and a fridge magnet tells you which you have in a few seconds.

How does it install?

Lift it up, let the magnets take it, with the closed side facing the direction you want blocked. That orientation is the entire function of the product, so it is worth double checking before you come down. Fit it, then go and sit where the complaint came from. If the air is still arriving, the closed face is probably 90 degrees out, and turning it is a few seconds' work.

Does this affect the rest of the system?

Less than any other configuration in the family. The outlet passes the same volume and still spreads it across three directions, so the branch sees close to what it saw before and the room keeps most of its original air pattern. That is the argument for trying this one first. Even across a floor, a set of 3-way covers changes less than the same number of solid or 1-way covers would.

Is this a filter?

No. It changes direction, not air quality. No filter media, no MERV rating, no filtration claim of any kind. Filtration covers are a separate product line.