3a fuses (sometimes written as 3-amp fuses) are common components in many types of household and workplace plugs and other electrical devices, both in the UK and elsewhere.
3a fuses, specifically, are most often associated with the sorts of household plugs used to supply power to smaller and less powerful appliances around the home and office.
In the UK, household plugs are often fitted either with a 3a fuse (if they supply a device or appliance that's relatively low-powered), or a 13a fuse (for more power-hungry equipment and larger machinery)
The general rule of thumb states that fuses will typically be rated according to the approximate power rating of the appliance they're protecting - this is why it's important to always check you're replacing a fuse with an equally rated one (assuming the one that just blew was in fact correctly rated for the device in question!)
Broadly speaking, the familiar type of red 3a fuses we've all seen in many household plugs and devices are usually advised for supplying appliances that draw up to around 700 watts from the wall - this might include equipment such as: