I'm not sure why, but I've played this game quite a bit and my titan only has 2 subclasses. (Void and Arc)
I'm lvl 20 *290 but I have yet to receive the relic or whatever it is that I need...
Any info or help is much appreciated. Here's an image of my full inventory. [u]https://imgur.com/a/TcYMQ[/u]
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Have you completed the story campaign with the character in question? I'm pretty sure I got all three of my third subclasses before I completed the campaign with each respective character.