I got 8,000 pounds and I lent 5,000 to my friend. I have only 3,000 pounds, so do I have to give Zakaah from all 8,000 pounds?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. 

If your friend is able to repay his debt to you when you ask him to do so without delay, then it is your duty to give Zakah on all your property (including such a debt) that has been under your possession for one lunar year. This debt, in fact, is a form of deposit.
But, if your friend is procrastinating or unable to repay, then you have to give the Zakah for such a debt for one year only as soon as you receive it.
Allaah knows best.