Actually, that's quite hard to decide whether it could be viewed as legal use or not in such case.
First, you don't buy the program, but only a license for its usage. Then it surely depends on the license being used for the particular program.
Also we can partly say, that when you use it for your own private purposes, it might be considered as legal usage, but in that case, you surely cannot publish your work anywhere (regardless if it's commercial or a mod). In some countries you are allowed to use a copy of the program for your own purposes, but it is supposed that you own also a legal copy of it.
And only the fact that the 3D program devs do not pursue modders really do not legalize any possible illegal program usage. It is just because they don't have enough capacities to hunt down every single pirate. (Just imagine Paradox paying hundreds of lawyers to make all the pirates pay for their games...).
Finally, I don't believe that only because you are using anything under authorship protection for educational purposes only would excuse such usage without the license. That would allow all the schools and other facilities like that to use e.g. Windows on their PCs without actually buying it (in real, they still need a multilicense). Actually, this is widely discussed dispute and can hardly be solved until the authorship laws get more clear in such case, so I would not be that much certain about it...