1. I think you can push yourself harder than a 3, but I wouldn't go all the way to a 10. I would workout at a moderate to hard level. You can sweat and breathe a little heavy. I push myself a little during running and I do yoga where I push myself a little.
2. I feel pressure from running (I'm 33 weeks) and I run through pressure and discomfort. I also have a maternity support band thing that helps.
3. I can't think of why stationary biking would not be okay. At this point, I don't have a lap, so I can't imagine it would be very comfortable on a bike with that belly all the way down my thighs, but early on, I can't see why not.
Re: Pushing myself
And yes- I did spinning until about 6 or 7 months- then when my belly popped I couldn't really- it was uncomfortable!