There are four ways to interpret your question:
compute the probability exactly when each length $\ell_{ij}$ is chosen independently
compute the probability exactly when 6 numbers are chosen independently and you have a freedom to assign them to edges in any way
you want some (say, 0.001) lower bound in either case
you want a 1/3 lower bound in either case
In case 1), consider the set of possible tetrahedron 6-tuples of edge lengths as a points in $\Bbb R^6$. You basically want to compute the volume of this set. Unfortunately, this set is non-convex and is defined by rather nasty inequalities (see this Rivin's paper which I already mentioned on this MO answer). To get convexity, Rivin shows you need to consider squares of edge lengths. In other words, the desired volume is a volume of an algebraic body and is likely be non-algebraic itself. It being 1/3 is doubtful.
For 2), the problem is much harder as you have various permutations to consider. For 3), this is easy - a small enough perturbation of lengths of a regular simplex will work. For 4), this may or may not be true. I sort of doubt it if you don't allow permutations, but with permutations I have no intuition. In principle, you can simply approximate the volume of a body in $\Bbb R^6$, there are better ways for doing that than sampling random points and checking if it's in there. Either way, it is hard to imagine how you would later extend this to simplices in higher dimension.
