You can only ever use one skill at a time. He would have to choose between his dodging and shields skill.
Your base dice pool is attribute + skill + equipment which is capped by max dice pool. Anything beyond that, exploits, modifiers, etc? All that is after.
I think i have it down now, Please correct me if I am wrong. you would choose to use shield skill for defense , so you would add one for that and then 1 for high quality shield, plus the 3 for agility, for 5d6 pool.
If you had 3 in shield skill and a high quality shield , Plus the 3d6 agility pool you would go to 6d6