Consider this scenario:

DAG1 with 3 MB servers.

2 Datacenters: 2 of the MB servers in datacenter A, 1 in datacenter B

datacenter A experiences an outage, and both MB servers in dc A goes down at the same time.

Can Exchange brought back online / failed over to datacenter B with just 1 MB server?

From what I'm reading about cluster quorum, DAG will not function if fewer than half +1 members are online. Even with dynamic quorum, if 2 MB servers go down simultaneously, quorum can't adjust (for dynamic quorum to work, each server needs to go offline one at a time sequentially).

One thing I heard was the only way to failover in this scenario is to remove DAG, and bring the MB in datacenter B as a standalone MB server...

I also saw something about enabling DAC (Datacenter Activation Coordination mode) for the DAG can mean I can failover Exchange to datacenter B with just 1 MB...

Any thoughts would be apprecia

You can force quorum by using Set-DatabaseAvailabilityGroup to tell the DAG that only one site is available.  When the other site is back, you can then restore service to the full DAG.  This procedure describes the steps in the following article.

https://technet.microsoft.com/en-us/library/dd351049(v=exchg.141).aspx