Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/STMicroelectronics/STM32U031/TIM3/TIM3_AF1#0x0
ETRSEL=B_0x0
TIM3 alternate function option register 1
ETR source selection
0 (B_0x0): ETR legacy mode
1 (B_0x1): COMP1 output
2 (B_0x2): COMP2 output
https://github.com/cmsis-svd/cmsis-svd-data