Indians and the number 3

Something that intrigues me about the census is the different codes they used: When someone was marked as married, for instance, but their spouse was not present, they crossed out the "M" and wrote a 7 above it. Digging through forum posts at Family Search informed me that that is what the 7 meant: separated from spouse, basically.

Here's one I haven't been able to figure out though. Native Americans (labeled as "Indians" in 1940) were always (at least in Oklahoma) marked with a "3" above the crossed-out "In."

From Washington Co, OK, ED 74-13, Sheet 6-A

This was a mixed-race marriage, notice. It was also unusual in that every other "Indian" family I've come across so far had all of the school-age children marked absent, apparently all away at "boarding schools" - part of the so-called "civilization or assimilation process."