Silver solder is lovely stuff, very easy to work with given a half decent ~50watt iron or hotter. You shouldnt need the flux, just make sure you clean and tin the tip of the iron before using it- best to pre-tin the leads and the bullets to make joining them easier too. The MMMv3 uses 6.5mm bullets on the esc itself, so best to use matching items on the motor leads (males)- extend the motor wires as required and insultate the joints with heatshrink tubing, color co-ordinated ofcourse for extra cool points...
Same deal with the battery wires, just use your favourite deans, trx or bullets etc and pre-tin them to make soldering quicker and easier.