keep creo 2 and 3

Yes, just install into a different root folder.

yes bro,.........

but u instel in different os. if u do not then it will hang ?

You can have both versions installed and they will both work (I have both installed right now).

Keep in mind though that they are not backwards compatible like most Microsoft Office products are. As soon as you save something in the newer version you will not be able to open it in an older version. The caveat to this statement is that there is a complicated plugin that does allow you to open newer files with the older versions but it does not restore full functionality. It only allows you to add features in the older version which you can then use in the newer version. It also doesn't always work correctly.

PTC Creo GRANITE Interoperability Kernel

you can installed both version  in one system, like w-7,8.1,8 , XP etc like ,then u can easilly work

Otherwise u uninstalled creo 2, then after installed creo 3, this is the batter way bro............

As others have said, there's no issue with installing multiple versions.  I have WF4, WF5 and Creo 2 on my machine right now.  All my work is in Creo 2 at the moment, but like you, I need to be able to use what my customer uses.

Some things to keep in mind, however.

1. Don't use the "double click to open" function to open Creo / Proe files.  The last version installed will be the one associated with Creo files and that one will launch, regardless of which version the file was saved in.
2. Starting with Creo, windows customization (tab layout, quick launch buttons, etc) are stored deep in the user's profile in the "cache".  Creo makes no distinction there between versions, so Creo 3 and Creo 2 will try to use the same windows customization file.  I'm not sure if this will cause issues or not, we set the cache folder for each version to avoid this.
3. Be aware of where your config files are stored.  If you are just storing them in the <load point>/text folder, then each will have its own files.  If you are using another folder, I'd give each version it's own config folder.  That's what we do.

yes