I know that MCP 3008 can provide sampling rate around 100Ksps. I'm looking to connect ADC with sampling rate least 3Msps.
If your ADC would operate with it's own memory you could operate it with RPI. Problem is not the RPI itself but the fact that you're probably not running RtOS on it.
More is explained in this post: Is Pi powerful enough for an oscilloscope project?
It depends on the ADC and how it stores/transmits samples.
You would need a SPI device as the I2C bus at 400 kbps simply doesn't have the bandwidth.
SPI you could drive at circa 30 Mbps.
However another limitation is in the Linux SPI driver itself. On the Pi you can only make circa 20 k calls per seconds to the SPI driver. If each call can only get one sample than you will be limited to 20 ksps, far short of the desired 3 Msps.
If you bypass the Linux SPI driver you might get 100 k calls per second, so 100 ksps if each call can only provide one sample.