I've been using the 3D heavy since the 22nd and I had no issues with the connection at all, don't know if it is local issue, try contacting Sprint maybe they are upgrading the network around you.
First thing I did if it wern't for the fun factor of the device I would have returned it already but im at a loss on what to do I guess we just have to wait for a update to hopefully fix this.
Ok just got my evo 3d and in the heart of Sprint coverage had great 3g and call signal in my location with the Evo 4g, now i don't have 3g coverage and i have seen better signal strength on Cricket Wireless. I don't know if the power was reduced to the antenna or what the problem is but fix it this is seriously messed up.
P.S. 4g coverage is 2 bars on both 4g and 3D