SQL server alwayson 3 Node Primary Down
Yes , it should work considering 3rd node is part of same windows failover cluster.
I believe with 2 nodes you are trying to achieve HA and if 3rd node is in different data center it's ideal for DR.
.
Also I am not exactly sure when you say taking DC down, if both nodes down, then test the case for quorum as I guess you might be on windows 2012 and things are tricky there for odd/even majority of votes