learning about modes!

so, we have finally come to modes. My guitar teacher started me off with this: for the I IV V structure in blues, we can use first position (minor) of the I chord, the fourth position (minor) of the key for IV for the IV chord, and the 3rd postition of the V chord for V. So, for A minor blues, it works like this: for I chord use 1st position of A, for IV chord use 4th position of D, and for the V chord use 3rd position of E. This keeps the hand around the fifth fret area -- these chord positions are chosen to be in the same area of the neck. Plus certain notes are emphasized in the IV and V chords sections, but I forget which. Something about the three notes they all share, like

A, D, and G or something.

Am I right about this?