will they all fire at once and deal the full 36 to the target?
Attack rolls from luck*weapon max (in the absence of luck, this is zero) to weapon max, so, no, not really. On average, 36 attack will do very close to 18 damage. In general, against a defenseless target, having 36 attack combined on 3 ships will give you a higher chance of doing x amount of damage than having 36 attack on one ship, but it's still going to average around 18 (theoretically, your mileage will vary for "short" battles").
You didn't answer my question, though-you say it "seems" like only one of your ships fired, which makes me think you didn't watch the battle screen, which makes me think you have it only set for fleet on fleet action...which this wasn't.
In DA/TA, though, even if your ships were destroyed in the first round (and ignoring the fact that you were the attacker), each one of them would get a chance to fire. How much damage they do is another thing entirely.
Your final question is a difficult one to answer, and it depends largely on what the DL ship is. If it has more attack than your ships, which is likely, then sending a single ship against it, you'll practically never win. (DA introduced a "tie rule" thanks to simulating simultaneous firing, whereby if after 50 rounds both sides are still alive, only the ship with the highest attack survives. For combat involving a fleet, this is 300 rounds, but the same principle applies.) However, it's also unlikely that the DL ship in question has a mere 2 weapons, so 3 ships isn't going to be enough. (If for instance the DL ship has 9 weapons, it can kill a maximum of 9 ships in the first round of combat, assuming it OHKOs all of them. As such, using ten ships will eventually defeat the tie rule.)
Almost forgot: Click on an enemy ship and click the little i button for the intelligence report. You can also get to it if it's a fleet by clicking details first.
