My laptop showing 3GB instead of 4GB

you need upgrade your OS to 64-bit.
http://msdn.microsoft.com/en-us/library/aa366778%28VS.85%29.aspx#physical_memory_limits_windows_7

On a 32-bit system, anything over 3GB will be allocated for graphics. See here: http://support.microsoft.com/?kbid&ID=929605 

Like thinkpads_user mentioned, upgrading to a 64-bit OS will fix the issue. Just make sure your CPU is compatible first.

Since you have windows 7 you could try enabling PAE in your boot config... http://msdn.microsoft.com/en-us/library/windows/desktop/aa366796(v=vs.85).aspx

This is common on a 32bit system and at most you will get upto 3.3. Usually though around 3 due to the addressing system. If youy want to increase the amount of RAM available to you then changed to a 64 bit OS.

"... please help me to resolve this issue. "  ==>   There's no issue to resolve.

Your laptop's working fine.     Your 32-bit OS can manage a 4GB address space;  but there are certain system level functions that must be assigned addresses before the RAM addressing is allocated.    BIOS shadowing; video aperture; PCI addressing; etc.     After those are assigned, all remaining addresses can be used for memory.    Typically that will be between 3.2 and 3.5GB, but on systems with a lot of video memory it can be less.    It would seem that in your case there's 2.99GB left.

You can see the exact allocation of your address space by going to Device Manager; and selecting View - Resources by Type - Memory
... this will show you the actual address assignments in your system

To use anything more than 3Gb needs 64-bits as I posted back at the beginning

@ramachandraraju  - It would be nice for you to follow up now.

.... Thinkpads_User

"... 32-bit can only manage 3Gb (plus or minus a wee bit). "  ==>  Not true.   It can address & manage a full 4GB address space.    How much of that can be RAM depends on how much the system level requirements needs out of that address space.    I've seen as much as 3.7GB available on systems with video adapters that had small memory complements;  and as little as 2.5GB on systems with a pair of 1GB cards in SLI or Crossfire.     It's certainly not true that "... To use anything more than 3GB needs 64-bit" ==> but it IS true that you won't be able to use all of an installed 4GB of RAM with a 32-bit OS.

I don't play with words. If you want 4Gb available for programs, then you need a 64-bit machine to do it. That is what I said way back, and then again above. I think you are saying just the same thing in your last sentence. .... Thinkpads_User