Pour 1/2-ounce of the tequila into a small saucer. Spread the kosher salt in a separate small saucer. Dip the rim of a martini or other wide rimmed glass into the tequila. Lift out of the tequila and hold upside down for 10 seconds to allow for slight evaporation. Next, dip the glass into the salt to coat the rim. Set aside.
Halve 2 of the limes, cut a thin slice for garnish from 1, and set aside. Juice the halved limes into the bottom of a Boston-style cocktail shaker. Cut the remaining 2 limes and the orange into quarters and add them to the juice in the shaker. Add the agave nectar to and muddle for 2 minutes until the juices are release. Strain the juice mixture through a cocktail strainer into the top of the shaker and discard the solids.
Return the juice to the bottom of the shaker, add the remaining 1 1/2 ounces of tequila and any remaining on the saucer. Add the ice to the shaker, cover and shake for 30 seconds. Strain the mixture through a cocktail strainer into the prepared glass, garnish with reserved lime slice, and serve immediately.