Canonical URL for a home page and trailing slashes

My home page could be potentially linked as:

(the same page is served for all those URLs)

I am thinking about defining a canonical URL to make sure Google doesn’t consider those urls to be different pages:

<link rel="canonical" href="/" /> (relative)
<link rel="canonical" href="" /> (trailing slash)
<link rel="canonical" href="" /> (no trailing slash)

Which one should be used? I would just slap / but messing with canonical seems like a scary business so I wanted double check first.

Is it a good idea at all for defining a canonical URL for a home page?