Q: How to download prepaid music

You've now paid for the album ? Pre-orders aren't paid-for at the time of pre-ordering, they are only purchased when they are released (if you have a balance or card on your account to pay for it). If you have paid for it then try the Account > Check For Available Downloads (Store > ... on older iTunes) menu option, or go to Account > Purchased or the Purchased link under Quick Links on the right-hand side of the iTunes store homepage

Do you mean that you've now paid for it ? The 3 songs would have been bought when you pre-ordered the album as they had been released early, you wouldn't have paid for the rest of the album at that time - only now that the album has been released would you have purchased (if you had a balance or card on your account) the rest of the album. Have you now paid purchased it, it now shows on your account's purchase history ?