I have 3 partition in my external hard drive, but I m not being able to open one of the partition, and everytime I try to open it, it is being asked to format
Unfortunately, most probably this partition got corrupted. You can try some partition recovery software. Check this out: http://www.tomshardware.co.uk/forum/280932-32-hard-drive-recovery
After that, I would advise you to test the drive with some brand specific testing tool. If the results show something wrong, it may be a good idea to back up the data stored on it.