I'm doing a 3 inch block lift on my cherokee, I was wondering if I have to need to get longer shocks, if so what size? 

what year is it my friend has one thats realativley new that he did the same thing to, ill ask him about it and get back to u.  

Definatley need new shocks, you will bottom them out easy. XJ's are know well enough that a shock comapny should be able to tell you what length you need. If not, find a ramp, drive as fare as you can up it with no shocks, measure the compressed side to get the min. length and the other side for the max length. Same results can be accomplished with a jack and blocks, but its not near as fun! These will be rough numbers but will be a good spot to start.  

its a '99.  

You definitely need new shocks. It's best for get with the people your'e buying the shcks from to get the correct model for your lift - as someone else mentioned here. They usually range from 2-4 inch lift, and from 4-6 inch lift travels.