Q: I have two OSX installed on my macbook pr, having 3 partitions.One for Maverics, 2nd for Yosemite, and 3rd for common storage. Now how to uninstall mavericks and install windows 7 ?

To uninstall Mavericks just boot to Yosemite and go to Disk Utility and format/erase the Mavericks partition.

To install Win 7:

Install Windows 7 and earlier on your Mac using Boot Camp - Apple Support

What year/model is your Mac? W7, on most Macs, uses MBR/BIOS which will not handle any Mac which has non-standard disk partitions. Each OSX installation can also  have it's own Recovery HD, which will cause further issues.

If you really want W7 using Bootcamp, your Mac should have standard configuration. If it is not possible, use a VM engine (Fusion, Parallels, VirtualBox) and forfeit the ability to access Mac hardware directly.