Arduino Mega with two Arduino Unos as slaves via I2C?
Welcome to the wonderful world of microcontrollers!  Looks like an exciting project, and you will definitely learn a lot!

As for wiring with I2C, you have a few things to connect:


SDA (the "data" line)
SCK (the "clock" line)
GND (ground)


For I2C (which is also called TWI for two-wire interface), you connect all like lines together.  This means connect all the SDA pins (analog pin 4 [A4] on Unos, digital pin 20 on the Mega) and all the SCK pins (analog pin 5 [A5] on the Unos, digital pin 21 on the Megas).  I think the schematic is different than this.  I can't read the actual lettering on the boards because the image resolution is not high enough.  You also want to connect all the ground pins together, which you've done in your drawing.

If you want a quick way to get up and running with I2C, use the example sketches in the Arduino programming environment.  Open up Arduino, then go to File > Examples > Wire.  In your case, I would pick one master and use the other two as slaves.  I would try using the master_writer on the Mega and slave_receiver on the 2 Unos.  A good idea would be to leave all of the accelerometers, potentiometers, etc. off to start.  Just have the master send a command to the two slaves, and when they receive it they can light up their on-board LED:

digitalWrite(13, HIGH)


Another note: the Fritzing sketch shows a 3.7V battery powering the Arduinos, but they need 6-20V and it's best to use 7-12V.  This may just be a filler battery you threw in for the drawing, I understand.  Just in case, I thought I'd mention it.

As suggested by Nick Gammon above, you will probably get the best response by breaking up the rest of your post into individual questions.  Good luck with your project!  Sounds like a fun one!