update: you cannot guard with a unit without a shield aka G3s may not be placed on guard circle unless thro QW

actually you can, its been FAQed on the official cardfight Q&A in http://cf-vanguard.com/en/howto/question/

Here's the question asked as well for convinience:

Q661 (27-06-2014)
Q. Can I call a grade 3 or grade 4 card without a shield to my guardian circle?
A. Yes, you can. As there is no shield, no power will be added to the attacked card. They are put to the drop zone at the same timing as cards with shield.