Would a Retainer Fix my 3mm Gap in my Two Tront Teeth? if So, What Type Would I Need? (photo)

Your gaps are too large to close with a retainer.  Look into invisalign or veneers.

You can close a gap with an active appliance up to 2mm. Anything larger may require braces.

A retainer alone will not do it.  You would need invisalign or traditional braces or veneers.........