Congratulations and welcome to CR! That having been said, look forward to you posting pictures!
BTW, I went from 300D to 5D3 and so I understand your delight! One word of caution - the DOF is very different. It takes a while to fully comprehend that... Also don't hesitate to push the ISO. It is not grainy at 800 I had a hard stop at 400 ISO. With the 5D3 even 12800 and 25600 ISO can be workable!
One more thing - if you think the images are soft even when you take a stationary subject from a tripod, try live view focus. If that is sharp and the normal AF is soft, you need to micro-adjust the AF for your lens.
Great comments! You have a high-performance machine with some great glass - I can't recommend FoCal enough to get the most from your investment... Like rpt says, don't think you have to shoot at 100 ISO to get good results... Mine goes through a battery quite a bit quicker than my XSi - I bought an extra for my bag...
Welcome and Enjoy!