Different cooking times and different dishes

I don't think theres a good way to do this if you're cooking them fully from the start. Souffles are usually best done just before serving. Plus, your oven needs to have the capacity to do all this.

The Au Gratin potatoes should probably be cooked most of the way (you don't specify the recipe you're using, so its hard to say anything more) beforehand, refrigerated, then reheated in the 350 F oven (for a usual casserole dish, I'd guess this would take 30-45 minutes?).

So, if you do the Au Gratin a day or two in advance, throw it in the fridge, start the ham at serving time - normal cooking time needed. Then, at serving time - 1 hour, throw the gratin in and put the souffle in at serving time - normal cooking time.

You might want to finish the Au Gratin with the broiler after the souffle has been pulled out, maybe with a drizzling of cream + cheese (maybe subtract a few minutes from serving time for this).

You can sometimes make tradeoffs like this, but there will be compromises. For instance, cooking your au gratin potatoes at 350°F means that the top probably won't caramelize nicely; you'll just have a dish of cheesy potatoes which could have been cooked on the stovetop. And, soufflés are pretty picky; one cooked at a different temperature probably won't rise properly, or may even collapse. The ham (assuming it's already cooked) can be cooked at a higher temperature, but it may burn on the outside before it's warm on the inside; a lower temperature and the outside may not brown.