Its based on pipelining concept which says :
Let the one packet travel from source to destination and thereafter every transmission time interval the next packet will also reach the destination..
So time taken by 1st packet = 3 * Tt(Data needs to be put into source end, router 1 and router 2) + 3 * Tp (There are 3 links in between)
= 3 * (packet size/bandwidth) + 3 * (link length/velocity of link)
Having found that , as said earlier now at each transmission time interval , one by one 999 packets will be received at destination end..
So for that we need = 999 Tt
So total time needed = 3Tp + 1002 Tt