well, you don't have a choice what impedance to run them at....they are 3ohm speakers and thats it. no ifs no ands no buts. Most headunits will drive a 3ohm speaker ok and amplifiers definetly will.
They rate the amplifiers at either 2 or 4ohm, but they will still run 3ohm.
Generally if the amp is 100wrmsx4 at 4ohm and 200wrms at 2ohm, then at 3ohm they'd be running ~150wrms.
I have 2 sets of those speakers (Soundstream Tarantula 6.5" Co-axial) running off the Soundstream Tarantula TRX4.640 which is 100wrmsx4 at 4ohm or 160wrms at 2ohm so it should be around the 130wrms mark at 3ohm (speakers are 120wrms).
The Xstream components aren't that great though imo, better off spending some extra for the Tarantula or Reference components depending on what you want (SPL vs. SQ).
Exactly as above. Theres a new Pro series also, but I haven't heard them personally and not sure on pricing (probably pretty pricey).
The Tarantula's are a great set, they are very loud when amped up and they do sound quite good. They're a louder speaker than the Reference, but the Reference are clearer in my opinion but won't take the beating that a Tarantula will.