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.