How to show just one door open in 3D?
If your Door Family was set up to allow for the 3D Door Panel to rotate, driven by an instance parameter, it would be simple and uncomplicated. Most Door families that I have seen are not set up that way, and the 3D Door Panel geometry is placed in the closed position, without a parameter to swing it.
Setting up a Door Family with a rotation parameter to allow you to "open" the Door can be done, but I would argue that doing so is not simple or uncomplicated. If you only need one door to be open, and only open to a specific, fixed amount, I would suggest that that the simple, somewhat uncomplicated path would be to make a copy of the Door Family (with a different name) and to rotate the 3D Door Panel by the desired amount, and then load that into your project and change that single door to the new family. This will likely not be as simple and uncomplicated as that sounds, as the existing 3D geometry of the Door Panel is likely constrained by parameters so that it can change to match various door opening widths and heights. You would have to break those constraints, and then, with the geometry in the open positition, re-establish them, so that the panel would still change to match the opening dimensions.
A somewhat more complicated alternative would be to modify your current family, leaving the existing Door Panel geometry in place, adding geometry for another panel in the open position, properly constrained, and then set up visibility parameters to allow you to, on an instance basis, choose to show the 3D Door Panel either closed or open.
