Do you still have the replay? 

pm ed u

This is the "Discharge Missile" special of the Kanrak Assailant.  It's a little bit deceptive since the beam is actually created by the missile when it hits its target, so it's not clear that it's coming from the Kanrak at all.  The "can't survive more than a few seconds" thing threw me off and made me think this was a bug, but reviewing the replay the issue was merely that the enemy had damage upgrades maxed out and outnumbered you, which is going to be a recipe for sudden death in most cases.

Strategically speaking, it appears you fell behind economically much earlier in the game and your relative fleet size suffered as a result.  Your defeat was a forgone conclusion long before these battles were fought.  This is a fairly common problem for new players that you'll overcome with a bit of experience.

My best guess would be that you're looking at the illuminator vessel, which has two beam weapons on its sides in addition to a primary beam weapon up front.  Hence, each illuminator fires three beams at a time and they will usually strike different targets.

There was a bug last version that caused the illuminator's beam weapons to deal additional damage, but if you're fully patched this should not be an issue (the illuminator is a mediocre unit currently)