dear wheni open 3gp file it always open with real player.how can i change its open with type to another software is there any way

Press right button of your mouse on the 3gp file and go to "open with". then select the program you want to use to open the file.

If you want to open that file on a S60 Device use the FileMan to set the properties and you can find the Open with option. Please Note you assign other Apps such as "Helix Or PVP Player" instead of the default Real Player.
If you want to open in a computer you can use the idea suggested by "Kavit"