Ethereum mining is still profitable on the 3GB cards, just keep in mind that as the Ethereum DAG file size grows, it will become too large to work on 3GB cards. Most estimates say the file will be too large sometime in the middle of 2018, so until then, the 3GB cards are a way to get ether mining hash power for about 2/3 the price of the 6GB cards.
Thanks to some savvy calculations over on Reddit, the Ethereum DAG file size will be larger than the 3GB memory on these GPUs (2.92 GB plus some overhead) at block number 6,750,000. Calculating the block times, it puts the estimate around November 12-18 in 2018. That’s likely when the ethereum mining with 3GB cards will no longer be possible.
On September 23, 2017, there was an Ethereum developer call. As of October 17, 2017, when block number 4,370,000 is mined, the Byzantium update will be released. There will be a change to the mining rewards from a 29 second block reward to 14.1 second blocks. The reward will be 3 ether for each block mined.