Many people think putting a washer in a tap will fix a tap sometimes it will, but after fitting 3 or 4 washers and the tap is still dripping this means the problem is not the washer
The truth of the matter is that the seating of the tap can be corroded and or out of alighnment with the valve so therefore the rubber of the valve is not sealing down evenly, hence the water will leak out even when 10 new washers are installed the tap can still leak.
This tap needs to be machined, but before doing so that the thickness of the seating brass needs to be checked with a micrometer otherwise the body of the tap will be permenantly destroyed.
If this is a breaching piece inside a wall it will need to be chopped out and a new one installed this could mean tiling as well possibly a days work.
The services of a proffessional are needed for this situation to be solved
The spindles need to be serviced with new O type rings and new grease as well.
Please check that you know the location of the water meter and that it is accessable without having to crawl in through the spiders to close off the water before calling a plumber, this can save you money and the plumbers time