What physical maintenance should I perform on a MacBook Pro?
As long as you use common sense and generally look the MacBooks, you should be right. There's no need to change parts until you need to (e.g. a hard drive fails, a bad memory module, etc). 

So, the number 1 thing I would recommend is to ensure you have a backup regime in place (e.g. by using Time Machine). So, invest in a number of external hard drives and regularly backup your data on all three MBPs.