Am I correct on this

For 321, there are 3 dps, 2 tanks, and 1 support. Wouldn’t that not be enough support?

No it’s 1-3-2. which is 1 tank, 3 dps, 2 supports.
1 Support is basically unplayable, as confirmed in that Dev post.

Jeff explaining it:

I would much rather have 222 than 132. I love combining rein and Zarya.