Yes, it fits. My friend put 16" rims for winter on his Speed3 2008
It just that there is very small clearance with the caliper and when you hit a pothhole, it may or may not damage something, but to answer your question, yes, you can fit 16" rims on a Speed3
It should just clear. I'm going to assume your calipers can't be any bigger than on a speed6. As mentioned above, you may maybe have some very tight clearance though.
I recommend you run 17in.
There is less then a hand full of 16in steelies that would clear the caliper.
In winter, I run 215/50/17. It matches the stock overall wheel diameter.