Welcome to the Forums!
And I am sorry your experience has not been more positive.
Unfortunately, we will need more information: please read the top sticky posting at the top of this webpage -- the one that says Please Read This Before Posting!!! Then get back with the information it mentions, and I am sure someone will be able to help.
In particular, we will need to know what format your .avi files really are. AVI is only a wrapper or carrier format, you see, and there are over 800 different types of video which use it, and their properties vary widely. More to the point, they use a wide variety of different codecs. So could you right click on at least one of the clips within Video Studio, either in the timeline or in the library pane, and copy down ALL its Properties here please.
I am unable to offer any advice on the .3gp clip. In fact, I didn't even know that Video Studio could handle that format. I have a phone which uses that format for its video, but I have never used the video function on it so don't know. [EDIT: I have just had a quick look at the formats which can be accepted by VS and did not see .3gp listed among them.]
One way or another, though, I suspect that you have a codec problem. As far as I am aware, you can only handle two types of .AVI with the codecs which come as part of Windows -- and that is uncompressed .AVI and DV/AVI from a mini DV video camera. With all the other AVI formats, you actually have to install the relevant codec which Video Studio can then use. Thus, for instance, if your AVIs are (as I suspect) DivX or XVid, you have to install the DivX codec or XVid codec. DivX has a free version on DivX.com, and XVid is an open source codec (also free) which you can find with a simple Google search.
The .3gp codec is probably a similar case, though hopefully someone who has experience with that particular format will comment on this.