The 3-2 program requires very good planning skills. And a lot of people who set out with the intention to complete it decide halfway through that they don't want to do it. Which is why there are normally no more than 1-2 people graduating with a dual degree per year.
The engineering program is not difficult to complete per se, and certainly a lot of people have done it, but many students simply lose interest. Also, I don't think any of Reed's partner institutions guarantee full financial aid for the final 2 years.
As for the GPA requirements, I think you're guaranteed to get into Columbia with a 3.0, while Caltech has its own selection process and Reed can only 'recommend' students for admittance; however, I can't imagine Caltech's GPA cutoff, if there is one, is higher than 3.3. There's no Reed-related program that I know of that asks for a GPA over 3.3. I think the RPI program isn't that hard to get into. I'm not sure about this, though, so you should check with the program coordinator at Reed.
By the way, a guy I know wants to go on to study materials science after undergrad, but he's perfectly happy getting a BA in physics first.