The iii chord has only one extension and the matching mode is phrygian, in this case B phrygian.
As you can see a knowledge of the modes can come in handy for this tutorial.
Hopefully the explanation in the previous video will help you to understand the relationship between the chords with their matching mode.
In phrygian we have 1 - b2 - b3 - 4 - 5 - b6 - b7 - (8 - b9 - b10 - 11 - 12 - b13 - b14)
(note that our 8, 10, 12, and 14 cannot be used as extensions because they are already being used an octave down as our 1 - 3 - 5 - 7)
We cannot use the b9 because it is a half step above a chord tone and the same goes with the b13.
Our 11 is the only extension that we have available for our iii chord.
Our IV chord is awesome because we have no avoid notes!
The IV chord's matching mode is lydian which is as follows:
1 - 2 - 3 - #4 - 5 - 6 - 7 - (8 - 9 - 10 - #11 - 12 - 13 - 14)
Extensions - 9, #11, & 13
Avoid notes - none
Extensions - 11
Avoid notes - b9 & b13