Sure, I'll provide you a replacement of U3. As you currently have an STM32F103RCT6, a suitable replacement should have similar parameters. Here is a good alternative:

Designator: U3_Alternate MPN: STM32F303CCT6 Manufacturer Name: STMicroelectronics

The STM32F303CCT6 is also an ARM-Cortex M4 microcontroller with an operating frequency of up to 72 MHz, similar to the STM32F103RCT6. The STM32F303CCT6 series focuses on motor control, medical applications, and applications requiring high analog performance thanks to its high analog integration.

It's important to note that it's not always a drop-in replacement due to different pinouts or additional features which might not be available on the previous part. Therefore you might need to make changes to your schematic and PCB design to accommodate this new part. Do you want advice on how to modify your project to accommodate this new part?

Yes

Sure, here's how you could adjust your project to accommodate the new STM32F303CCT6:

Truth be told, direct replacement may often require a new board revision, as pin compatibility between microcontrollers from the same family (or even different families from the same manufacturer) is not guaranteed.

Just a reminder: Always verify these changes using the datasheets for each component, especially as this is more of an "in-principle" replacement suggestion.