Yes they are. Bioware stated that the ending choices are canon in the Mass Effect Universe. So by Word of God, they are canon.
Let me explain this in the most simplest way possibly...
You cannot have 3 different outcomes from one event as "all Canon" only one outcome can be chosen which then becomes canon.
Another example
If there was a ME game set one year after ME3 and all 3 outcomes were canon that would mean all life in the galaxy would be dead as synthesis and destroy would cause the death off all life.
Synthesis makes all organics and synthetics merge.
Destroy kills all synthetics.
Not to mention both Control and Synthesis require the death of Shepard in different ways.
There is only one Shepard so how can two different deaths happen to the one person?
They can't all be canon.
Please understand the terminology