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It depends on what type of concrete you have and the conditions under which it is curing. First, it's imperitive that the soils underneath the slab are well compacted. If they're not, the slab will probably settle in the uncompacted area when heavily loaded (like with a D6 on it), causing cracks. Secondly, the strength of the concrete at 3 weeks depends on a few things. The psi rating and thickness of the concrete are the biggest factors in its strength. Is it 2500psi, 3500psi, 5000psi? Most residential concrete is 2000-2500psi @ 4" thick unless you've specified stronger concrete. I'm not saying 2500psi at 5" thick wouldn't hold the D6 ok, but I know it's common for shops dealing with heavy equipment to use stronger than 2500psi concrete in their slabs. Secondly, 3 weeks is a bit early to put the D6 on it in my opinion. Ideally you'd want the slab to cure under ideal conditions for at least a month (and preferably longer) in order for it to reach near optimal strength. Even then it will not have reached it's max strength. Moist cured concrete's strength increases rapidly initially and then slowly increases over time to its full strength over several months (reaching about 75% of its max strength at 1 month if memory serves). Moist curing is substantially stronger than dry cured as evidenced by the graph shown below. Also, did you have an inspector monitor the pour when it happened? It's common for unsupervised pours to add too much water to the concrete during the pour. The reason the workers add excess water is because it makes the concrete flow better and it's much easier to rake and work (working concrete is backbreaking work!). In this day and age it's very common for immigrant Mexican labor to be doing the concrete work too, and while they are very hard workers, they're often used to how things are done in Mexico (where there are few/no inspectors). So you'll hear them yelling "agua! agua!" while making the "drink water" sign to the concrete truck guy trying to get him to "soup it up" for them. But again, the only problem with excess water is that it greatly weakens the finished concrete (i.e. if you ordered 2500psi concrete, you could effectively end up with 1700psi concrete with too much water). Too much water causes the concrete to shrink excessively and crack as it cures. Concrete should usually be poured with a 4" slump unless otherwise specified by the engineer (concrete "slump" is measured with a slump cone). 
Re moist cured: Ideally, the slab should have been covered with plastic (or wet straw) and kept wet the whole time during the 1 month long curing process. Doing so makes the concrete cure to a significantly higher strength than if it's simply left uncovered/unhydrated. It should also not be subjected to freezing conditions, otherwise it will be weakened and probably crack. Insulating blankets are usually put over the slab in freezing conditions (heaters can also be used if necessary). Bottom line at this point, is I would wait at least another week before driving the D6 on the slab. Then put heavy boards down where the tracks are going to set in order to keep the grousers from chipping up the new concrete. Good luck.
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