I'll chime in with some thoughts, though I personally have little experience with these type of designs (brand and price, lots of items per task, large respondent size). I'm writing this under the assumption that when you say 70 items, you mean something like 70 SKUs in attribute 1.
Regarding the number of items per screen, that's something that falls under the human side of research, rather than some sort of mathematical calculation. You need to do some pretesting and make sure the task does not overwhelm someone. There's really no right or wrong number of items, just trade-offs that you make. More items per task results in a more efficient task, but without any additional information, I feel like 30 products on a screen might be a bit overwhelming for someone to really consider all of them, and might just be hard to fit all of those things on the screen without requiring the respondent to scroll around.
I picked up a rule of thumb from someone a lot smarter than me that it's a good idea to have levels showing in your design 5-6 times when using HB. I don't have any math or anything to back it up, but it seems to work well. In your case showing 30 from 70 would result in a few levels only showing up maybe once or twice, which feels a little sparse. On the other hand, 3000 respondents is a pretty big sample size going into HB, so here's where I reference my inexperience in these types of designs and avoid speculating on just how this would turn out. Perhaps someone else who does more of these types of studies would like to chime in.
One other note is that a typical CBC exercise usually is in the neighborhood of 12-16 tasks, and we've got a paper or two showing that people can reliably answer 20 tasks. Again, it's a bit of a different thing when you have a lot of really simply things on the screen, but I wouldn't be afraid of going higher than 6 tasks for the respondents.