Thats something that really can't be predicted. Each site uses a different amount of RAM. 3000 users online is quite a lot, but 512MB of ram should be able to pull it off. I'd be more concerned about the CPU.
Depending on the VPS system used, the memory can be limited by RAM or by Virtual Memory (Virtual memory is composed of RAM and Swap). How much memory each of your visitors will use is not easy to calculate and varies a lot, from programming to optmization to processing...
3000 concurrent users will also use a lot of CPU, so you do have to be careful to know that the machine will not be overloaded.
For that many visitors, I strongly suggest you to get a dedicated server.