'N3' refers to the connector type for the remotes... It is a proprietary Canon standard, and Canon remotes which are compatible with the 60D have 'N3' in their model numbers, specifically:
- Canon Remote Switch (RS-80N3)
- Timer Remote Controller (TC-80N3)
There are also several companies that make N3 compatible wired remotes, including offerings from Adidt, Nova Photography, and Seculine. Generally speaking these tend to be cheaper (sometimes significantly so) but as you might imagine, quality can range from excellent to abysmal, so look for reviews before buying off-brand.