I have a set of architect drawn plans for a house. There's an A-Frame
section in the middle with vaulted ceiling (i.e. no 2nd floor) in the front
and a 2nd floor loft in the back. The plans specify the use of 3x10s on 12"
centers. These would be 23ft long with no support other than at the ends.
Seems to me that's pushing it a bit too far and the floor for the loft will
be very spongy. Any thoughts?

That span, even with 12" spacing, is long enough so that choosing
merely typical lumber for the joists will make it fail to meet a
30PSF live load specification. You're right, it's too bouncy.
Boise-Cascade's AJS-25, 9 3/4" I-joist is good for 22'11",
assuming L/360 and 12" spacing, and the solid lumber tables
vary from 22'9" to 15'9", depending on what you're using.

For your purposes, you want spans of no more than 18' for 2x10s,
or 22' for 2x12s. Are the floor-to-ceiling distances small enough
so that you're clawing for vertical space? If not, you should
probably switch to 12" I-joists, like
Boise-Cascades (AJS-25, 11 7/8")