I think you're going to have trouble finding any microcontoller with ADC sampling that fast . . . I checked the PSoC5 datasheet and it appears the make speed is 384k. The PSoC6 datasheet still looks a bit preliminary but there's nothing in there that seems to suggest it can handle that kind of speed. I think you're not going to have any choice except to use an external ADC.
I was able to partially reproduce approach posted by <kabron>,
and to overclock SAR_ADC to 1.88MHz sampling rate.
Unfortunately, digitized output amplitude drops quickly with frequency due to low analog bandwidth of the ADC and input path. For example, 500kHz sine digitized output drops to about 1/2 of input amplitude.
Secondly, gains for SAR_0 and SAR_1 differ by about 30%, making interleaved sampling problematic.
Lastly, PSoC must have XTAL installed to get to 80MHz clock speed to have desired 26.6MHz frequency fo ADC clock. Without XTAL, maximum bus frequency is 79.2MHz, and the overclocking trick does not work.
Without XTAL, it is possible to overclock ADC using bus frequency 53MHz and divider of 2, but only one of ADCs will work at a time, while being very sensitive to clock phase relative to bus clock.
Overall impression of that the effort is not worth the troubles. There are other chips with native 3MHz SAR_ADCs, else use external part.