There are two ways you can accomplish this: you can either use an online service which is very convenient because there is no interaction from your side in the converting process or use a 3rd party application which you install on your computer.
For the online service, there are thousands of websites which allow you to perform this operation.
On the PC you can use, for example, ImTOO YouTube Video Converter because it has the ability do download videos from Youtube in High-Quality mode and then convert them to 3GP or a format you want. Simply paste the URL of the video in the URL box and wait for the process to finish.
Please take a look at the image below to see what I meant.