Size 3 is the medium size in the Fosta Neoprene Ankle Support 413-3 range. Sizing for this type of slip-on sleeve is usually based on ankle circumference, measured just above the ankle bone. To get a reliable measurement, sit down, keep the foot at a roughly 90-degree angle to the leg, and wrap a flexible measuring tape around the narrowest part of the ankle, normally just above the bony prominence. Do this over a thin sock if that is how you plan to wear it.
Once you have the measurement, compare it with the manufacturer's size chart for this model. If your measurement falls between two sizes, the brand's guidance should tell you which direction to go. A very snug fit is normal for neoprene, but the sleeve should never pinch, cut off circulation, or leave deep grooves. Because the heel and toe are open, the fit around the arch and the front of the ankle matters more than total foot length. If you are between sizes or have unusually high arches, a podiatrist or the seller's product support can help you decide.
After you start wearing the sleeve, check the fit after a few minutes of walking. Mild compression feels like a firm, even hug around the ankle. If you feel throbbing, tingling, or see red marks that do not fade quickly, the size is likely too small, and you should stop using it.