What if one planet could have one player of each race inhabit it?
 
For instance, if you have a Terran, a Xerj, and a Protoss account, share one planet between the three accounts. 
The resource income would be divided between the two or three players, but all the defense would be combined.

No it s nonsense 

As all the players has the ability to have 3 accounts: Terran, a Xerj, and a Toss; why we don't have one console for that, as same as you can jump from a planet to other you can see easy all your species without Log out then Log in. In that way can clear all the multi account user then play in a fair way.

For a future development can find a way to combine the technology then can use a multiple advantages as a free arbiter we can create fully different species. with a mix of troupes and technology.

whether you go against a player with 3 species or a player with friends attacks - the result is the same ..... If the attacker did the calculation correctly, the defender will definitely lose the fleet and resources....
For the attacker to succeed, only the rules "speed" and "mass" apply ..... Technology only supports fuel consumption .....

So it makes no difference when it comes to fairness ..... the only thing that protects is to hide the fleet and resources - take certain security measures and be very attentive (watchful) in the game ..... This is the only way to reduce losses ..... If an attacker finds nothing profitable - he does not come (and it does not matter how many friends and accounts he has) .....