Switch for low voltage, high current Shape Memory wire
If you have a 2V supply and your control signal is (say) 5V you can just use a beefy MOSFET. 

For example, you could use an IRL40B209 which has a guaranteed Rds(on) of &lt;1.6m\$\Omega\$ with 4.5V drive (Tj=25°C). That means it would drop only 5mV at 3A. 
That one is probably overkill (though not so unreasonably priced unless you're into vast quantity), but you get the idea. 

If you need to create the 2V@3A supply from (say) 5V you could use a buck converter and simply switch the converter off and on using an enable pin or by jimmying the feedback voltage (assuming only one wire is used). A perfect buck converter would draw 1.2A to get your required 6W, so a real one might be more like 1.5A. 

An H-bridge is typically pointless in switching a resistive load because polarity does not matter and you're doubling the losses and quadrupling the power device count for no good reason. 

You could also directly PWM from a 5V supply (no inductor), however it would be drawing current in >8A pulses (which the supply would have to handle- peak power exceeding 40W). The on ratio would be 16% to get 2V RMS across the load from a 5V supply. 

Any of the above methods would work.