Based on the overall span, I don't see how 3 sections would do it and not being overly bouncy, and eventually sagging over time. I suppose, if you did it as you say, using overly long spans, and you concluded that it was not supported well enough, you could always go back and add more supports.
I am also cautious to agree with your idea to use a power auger to drive the posts knowing that at some point the points are sure to grab. I subscribe to the K.I.S.S. methodology; just spin them in by hand first and see how that goes before making the task more complicated than it may require.
I can use 2X10 joists if this will be better. Like you said I can add more supports later on or maybe just not secure the sections to pipes so they will slide up and down on the supports? And just put floating supports under the middle section?
Also I will try a simple handle first to drive it in.
