Here are some tools that would do the trick.
1.Movavi: I would say Movavi Online Video Converter is the easiest way to get your videos in the format you want. Steps to follow are simple.. go to the website, then either fetch it the URL of the video file (eg. This from YouTube. http://www.youtube.com/watch?v=8us3ZOPo3MU ) or alternatively browse/attach an existing file from your computer.
Then select the download format (this site has almost all formats) Then hit "Convert". Soon you would get a converted link to download the file... as simple as that. Try it to know its simplicity.
2.KickYouTube: Its the best way to download video files from the YouTube website. KickYouTube is a clone site of YouTube with a "kick" feature. This "kick" directly allows you to download videos in your required format, be it 3GP, AVI, MP4, etc... All YouTube files are available at Kickyoutube site.
Here is an video demo of the kick effect