Step 1: Download YouTube to 3GP software. Install and launch it.
Step 2: Download YouTube Videos and save YouTube flv to your personal computer:

Step 3: Convert YouTube flv videos to 3GP compatible video:
Click
to input YouTube video file to be convertered (batch convertsion supported, you can add as many YouTube files as you want).
Profiles option, you can choose "3GPP - 3rd Generation Partnership Project (.3gp)" or for 3g2 need "3GPP2 - 3rd Generation Partnership Project 2(.3g2)" to convert YouTube to 3GP:

Then click the folder button, select the destination folder where you want the converted 3GP video files to be stored.

Click
button to start converting YouTube to 3GP.
Ok! we have finished the whole process to convert YouTube to 3GP, transfer YouTube to your 3GP now.