Very, very different sound.
The Maggies can sound darn good when set up properly and driven with good electronics, but I wouldn't call them very detailed. They can be very musical and enjoyable, but don't have the same level of resolution as the HT3's.
One thing you will lose in going to the HT3's vs. the bigger Maggies is the height of the soundstage. Since the Maggies are so much taller, they will have a taller soundstage and make things seem bigger.
While I have never heard the 20.1's, I have owned 1.6's, 2.7's, and listened many, many times to the 3.6's in different settings - I wouldn't give up my HT3's for any of them!!