You can run ~1000 users if your RAM is 3GB. To run more number of Users Run Jmeter in NOn-GUI mode with Distributed Testing.
Please visit the following URL for detailed explanation. Ask me still if you get any difficulties.
While it is technically feasible to run lots of users performing lots of operations on a single machine, this is rarely a good idea (unless the system running the load test has greater performance characteristics than the system being tested). As you increase load on your computer running the test, it has fewer resources available to create the load on the remote computer.
What you really need to do is monitor all your resources on the computer's running the load test. Once you see any contention for CPU or RAM or HDD then you need to swap to distributed testing.