Of the three, a dumerils is hands down the easiest to care for. Rainbows need super high humidity without a wet cage, and they overheat (and die) quite easily.

Tree boas are also more care-intensive, often aggressive (and what teeth!) and quite expensive.

Dumerils can top 7' and 20lbs (I had a female over that size) but 5-6' is more normal. They're usually quite docile and easy to keep.