I have 3 different units running on 3 different stm32 microcontrollers, with all 3 sharing same ground, and I want all 3 boot pins connected together, to enable me to put them all into boot mode with the same input pulse. Is this connection ideal?

[Tesla DeLorean]

>>stm32 microcontrollers

Which ones, narrow it down a bit, as there's hundreds, in all sorts of pin configurations.

If just the BOOT0 (INPUT) pin, that isn't collocated with any other functionality / bond-out, then yes they can be driven by a common source.