At the boundary between analog and digital, you typically need to provide an anti-aliasing filter.
If you are measuring a differential signal with a large high-frequency common-mode component, then it also makes sense to perform the subtraction in analog. Otherwise the inevitable small phase shift between the sampling of the two channels in your ADC will end up coupling the common-mode signal into your difference.
At 3 GHz, the OpAmp would only offer a gain of 6 which isn't really enough for much accuracy.
Granted, the OpAmp is like $5 and the ADC is hundreds of dollars (and the ADC is only 7-bit accurate)... but the digital world has gotten scary fast and scary good.
That's why Oscilloscopes, even GHz-Oscilloscopes are being made with digital technology today.
26Giga-samples (Nyquist of 13). I mean, 3-bits sucks, but holy crap is that fast. A 26GHz ADC would be able to perform digital-filter analysis on 2.4GHz Bluetooth and Wifi without any aliasing what-so-ever.
Like anything, its a cost tradeoff. DSP can be much cheaper than active analog for many frequency ranges. You can certainly oversample your way to a cheaper antialiasing filter in many applications. But you can't ever eliminate it entirely.
If you are measuring a differential signal with a large high-frequency common-mode component, then it also makes sense to perform the subtraction in analog. Otherwise the inevitable small phase shift between the sampling of the two channels in your ADC will end up coupling the common-mode signal into your difference.