thay are all the same company but carry different stock. tesco extra is massive and you can buy almost anything there - furniture, TVs, clothes, painting + decorating equipment, toys etc etc. metro and express are, i think, in smaller shops in cities, on high streets etc.
be aware that you often pay more for the privilege of buying something in, say, an extra store, than you would in plain old tesco. i believe the same applies to metro and extra.