Had a hard time finding info on the forums on Invasion 3. Managed to figure the following from youtube videos and trying it myself. Just putting down a miniguide for the noobs like me that have no clue.
Prep: I would say largely stack lightning resist first (as I don't know how to reliably avoid swirly lightning), then maybe fire resists (to survive long enough to run into blue circle).
These are the boss skills:
1) Swirly lightning (like invasion 1/2) - largely tankable/healable.
2) Lightning circle (like invasion 1/2) - summons a lightning bolt from sky within the circle after some delay. If you stand within circle when bolt lands, you are prob dead. This is the skill to dodge for the bonus quest.
3) Frozen shambler - randomly summons a frozen shambler and a blue circle. For the first 5 seconds after being summoned, boss emits a strong holy fire aura. Quickly run into the blue circle to become immune to the aura. Stay inside till circle disappears. Shambler itself does neglible attack damage
4) Nova attack - boss will become immune to damage (as your indicator) for a few seconds before this attack. Quickly run into melee range to avoid this attack. You can be lazy mode, and stand in melee range for most of the fight, and just run out of melee range when he uses lightning circle/summons shambler. But the videos I've seen, people just run around freely then blink towards boss when he 'charges up the attack'.
TLDR noob tactics: stack max light/fire res. avoid lightning circle. run into blue circle when shambler spawns. stand in melee range of boss.
For the pros - please add on. I sometimes randomly still die to the nova attack though I am in melee range. Maybe I'm just outside the safe zone. I'm sure people want to find the fastest/safest way to farm Invasion 3, to access labs.