Dear Jin,
My name is Marco and I understand that are 3 WiFi purchases have disappeared. I am sorry for any inconvenience this issue may have caused.
In an effort to resolve this issue, I have posted the songs back to your account. Please follow these steps to start the download:
1) Open iTunes.
2) From the pull-down Store menu at the top, select Check for Purchases. If you can't find this option, you can also click this link to Check for Purchases:
https://phobos.apple.com/WebObjects/MZFinance.woa/wa/checkForPurchases
3) Enter your account name and password, then click the Check button.
Your missing WiFi purchases should begin downloading and appear in your Purchased playlist. If the download process is interrupted for any reason, it should resume once you reopen iTunes.
This article provides information about resuming interrupted downloads:
http://www.info.apple.com/kbnum/n93015
You can transfer the items you've purchased on your iPod Touch to your computer by following the instructions in this article:
Copying iTunes Store purchases from your iPod to a computer
http://docs.info.apple.com/article.html?artnum=305465
Thank you for using iTunes, Jin. Enjoy those songs and have a wonderful day!
Warmest regards,
Marco
iTunes Store Customer Support