Determine if two outlets are on the same circuit without breaker access
There are a few ways.  Let's assume you've already mapped circuit 1 and exclude that from these tests. 



This method will work if there's a long wiring run back to the service panel (breaker box).  Remove every big load from every circuit in the house.  Nothing in the house but wall-warts, vampire loads, and clocks. 

Now pick any receptacle, designate this circuit 2.  Plug a power strip into one outlet, and your biggest load in the other.  Put even more stuff on the power strip until you've loaded that circuit as much as you dare. Measure the voltage between hot and neutral as you add loads; ideally it is starting to sag a volt or two.  

Now measure the voltage between neutral and ground on every outlet.  We're looking for a small difference of less than 1 volt. Those are all on the same circuit - the one you loaded to max.  You will notice that some outlets have slightly higher voltage than others because they are at different places along the cable.  What they will have in common is their voltage will restore to normal when the circuit is unloaded.  

Now turn your loads off ASAP.  Breakers can take up to 20 minutes to trip if they are only slightly overloaded. 

Why does this work?  If you load a circuit heavily, its voltage will sag very slightly, due to our dear friend Mr. Ohm.  But hot and neutral wires are the same size.  That means both of them will sag about equally - hot will drop in voltage and neutral will rise in voltage.  Ground isn't sagging: like the unloaded indicator beam on a torque wrench, it's a reference point.  If ground tracks precisely with neutral, you may have a bootlegged ground. 



This method will work only if the two remaining circuits are on -opposite- poles in the service panel (assuming the usual 120/240 split phase service common to North America). All things being equal there's a 50/50 chance this is true. 

Pick any outlet, designate this circuit 2.  Plug a long extension cord into it.  Bring the extension cord near every other outlet, and measure the voltage between hot (on the extension cord) and hot (on the outlet).  Some will be 0 volts.  Others MAY be 240 volts.  If they are, you have identified your third circuit.  If you get 120 volts, something is wrong - e.g. hot and neutral reversed. If you get 0 volts the two outlets are on the same circuit.