So, I might be over-explaining, but I just wanted to elaborate on things a bit.
At level 3 you can pick whichever totem you want, then at levels 6 and 14 you can pick separate options from those new options, and aren't trapped with whatever spirit you chose at level 3. However, you're still limited at each level to only choose from that level's skillsets. You can't take Spirit of the Bear at level 3 and then go back and grab the Spirit of the Wolf at level 6... at level 6 you can only choose from the Aspect of the Beast options.
So, can you mix the totems? SCAG suggests you can with a couple of the recommended tribes, but can you do that without homebrew rules?
Yes. Each time you unlock a new subclass feature with a selection, you can choose whichever animal totem you want from the feature's list.
So, I might be over-explaining, but I just wanted to elaborate on things a bit.
At level 3 you can pick whichever totem you want, then at levels 6 and 14 you can pick separate options from those new options, and aren't trapped with whatever spirit you chose at level 3. However, you're still limited at each level to only choose from that level's skillsets. You can't take Spirit of the Bear at level 3 and then go back and grab the Spirit of the Wolf at level 6... at level 6 you can only choose from the Aspect of the Beast options.
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