TIe everything to the same apple ID and you can share all that's sharable. Bear in mind that while most iPhone apps work on the iPad (with varying degrees of 'looking good'), there are iPad apps that will not scale down for the iPhone.
And you may have little to no crossover with your Mac. THe Mac and the iDevices run on totally different operating systems so it's impossible for them to share apps.
Your music however, should be universally shared. Using the same Apple ID is key to sharing content.