Great question. My last effort to use a Tolassian Tome on a Companion meant THAT Companion got the Trait point, but not the other two, but that was so very long ago.
There are a few situations when using a Tome a Companion makes sense, namely when the trait point allows bumping up a Companion's trait to a critical level (say meeting the INT requirement to use Circle of Restoration) -AND-- the main Character has no real need of one more trait point (traits maxed, or too high for one trait point to make a difference).
But, I haven't tried that in a long time, not sure if it will work the same.