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Post: What you have described is correct. You should be able to power all of the boards from the GPIO header on the Raspberry Pi. A Pi 3 will normally draw between 280mA and 1A depending on the load and the expansion boards will not draw more than half an amp combined so a 6A supply will be more than adequate. With the Servo Pi boards if you are using them to drive RC servos then it may be worth running separate power cables from the 5V supply to the 5V inputs on the Servo Pi and have the power jumper on the boards disconnected. Servos can draw quite a bit of current when they move so by using a separate power cable the current will go down that cable from the power supply and not through the Raspberry Pi which will reduce the chance of voltage drops.