Oh so the T-nut has those 3 holes and I basically sink the tee nut a little, then nail it in?
To be repetitively redundant with my earlier post
here in the thread, YES!!

If you're using a forstner or paddle bit, drill out the countersink area then drill the center hole the rest of the way through.
If you're using a router, drill the center hole first, drop in the tee nut, trace the flange so you know what area to countersink, remove the tee nut, and route out the countersink.
Don't try to use a regular large drill bit to remove the countersink because the bottom of the hole will be sloped instead of flat.
The tee-nuts are just below the blue countersink area of this cutaway diagram.

Scott