I think what you need is going to take some sweat and tears on your part to accomplish with MODx. Depending on how good you are at coding, you can pretty much twist anything out of MODx, but I can’t say for sure that there currently exists a foolproof working integration between MODx and any external forum system. The SMF bridge seems to stymie most people, I haven’t tried it yet. For blogging, you can use pubkit but if you’re looking to build something the caliber of Wordpress you’re in for some more sweat and tears.
One thing you could consider is looking at systems that already integrate those three things in one, the first one that comes to mind for me is Drupal because I use it often. It has an inbuilt forum (nothing fancy but it’s functional) and inbuilt multi-user blog, all tightly integrated into the core.
Again, you can do this with MODx, all things are possible with MODx

, just depends on what you’re willing to put it in terms of sweat equity and what your coding skills are.