Problem with H bridge and Arduino
There are several problems here:

Darlingtons are a poor choice since they have relatively high voltage drop when on.  At only 15 V supply, the 1 V or so across each darlington is significant.

Not only will the voltage drop of the darlingtons rob voltage from the load, but it also causes the transistors to dissipate power.  At 1 V across and 3 A thru a transistor, it will dissipate 3 W.  That will require heat sinking.

At 3 A of load current, the transistors will waste about 6 W total.

Emitter follower makes no sense for the top transistors.  You want this H bridge to work with up to 15 V supply and to control it with 5 V logic signals.

The emitter followers as you show them provide current gain but no voltage gain.  Even worse, they will drop about 1.4 V from input to output.  With 5 V in, you won't get more than about 3.6 V out at the emitter.