When will Ethereium stop being possible to mine with 3GB GPUs ?

Around end of this year. But PoW phase of mining will finish before then. It will change to PoS before then.

I heard it was like in 45 days.

Where do you hear about it? PoW will last for around 10 months, then it will be replaced by PoS.

Ethereum forum, they made a calculation and said most GPUs won't be able to mine Ethereum due to DAG limit. Only 4GB and higher will work.