You can use a current mirror to actively and accurately divide the power load between your three DC-DC boosters. All you need is just one NPN transistor for each booster. I have used three cheap TIP31s, but any medium power type would do, as long as they are all the exact same model. This is because a current mirror depends on all transistors having very simmilar properties regarding beta, thermal behaviour, etc.
Building the current mirror is very easy: connect the collector od Q1 to the output od booster 1, collector od Q2 to output of booster 2, and collector of Q3 to to output of booster 3.
The emittors of Q1,Q2,Q3 are to be tied together. This is your new combined power output, going to the input of your load.
Next, connect all the bases together, and then - this is important - connect the base of Q1 also to the collector of Q1. Do this last step only with Q1.
Q2 and Q3 (being the same model and sharing a common base with Q1, will therefore behave nearly identically and copy the the collector-emitter current of Q1, no matter how much higher the booster 1 and booster 2 voltages are.
Lastly, you need to adjust your booster output voltage to compensate for one diode drop (because your transistors are now emitter followers).
For 12v output, set booster 1 to 12.6v and booster 2 and 3 to 13v (they need to be equal or higher than booster 1 to be able to copy Q1's current, but this will not affect your output at all, because it is now actively controlled.)
So, what have we gained from this approach? Same current through all parallel boosters times same output voltage (emitters are tied together) equals exactly same power delivered by each booster, no matter what the load, and no matter what the (higher) voltage of booster 2 and 3. Ohms law. And unlike just using resistors, the output voltage will remain nicely constant as regulated by Booster 1, no matter what the current draw of your load is.
I hope this is helpful.