giving MBP priority for Siri when iPhone and iPad are close by

Your Apple devices actually negotiate between them to decide which device is closest to the speaker, and there is no way to control which device gets priority.

As you suggest, if you flip over your iOS devices, they will not respond to 'Hey Siri' (this can be changed in Accessibility settings) which is the only control you have over the situation. The Mac has similar controls if the lid is closed. Also, if you have bluetooth off on any of the devices, they are not part of the negotiations, as they communicate with each other via Bluetooth.

For details refer to this Apple Support document: https://support.apple.com/en-us/HT208472