When does ETH will switch to POS with Casper ?
when is the POS?and my 3gb mining card will end in what month?
There is no official ETA at the moment for POS consensus implementation in the ETH blockchain.
As for 3GB DAG limit. I'm afraid it's very near now. Like a month if you're using Linux. People mining using 3GB GPUs are already facing issues with DAG file limit in Windows.
You don't guess on Ethereum Network - run the software in benchmark mode and set it to Epochs for later this year.
It will tell you at what Epoch, the Dag file is more than 3GB of VRAM on Linux.