Choose x1(0)=x2(0)=x3(0)=0 as initial conditions for x1, x2 and x3.
Then your constraint is satisfied.
And my proposition is that if
[a1 a3 a2], [a2,a1,a3] and [a3 a2 a1] are linearly independent vectors, this is the only solution.
Best wishes
Torsten.