The Digital Picture put a set of R3 test shots into its resolution/Noise/ISO comparison page. The results show that the new, low-resolution shooter to be shipped later this month very slightly improves noise performance at base ISO at the expense of providing less resolution and sharpness versus the R5.
The left side of the image comes from the R5 and the right side from the R3. The images were sampled to an equivalent resolution.
The Digital Picture’s comparison engine allows these bodies’ noise performance to be compared against any other Canon body, and to be done so at various ISOs and with both under- and over-exposed settings.