  The uncertainty principle is considered to be one of the most striking
features in quantum mechanics. In the textbook literature, uncertainty
relations usually refer to the preparation uncertainty which imposes a
limitation on the spread of measurement outcomes for a pair of non-commuting
observables. In this work, we study the preparation uncertainty for the angular
momentum, especially in the spin-1/2 representation. We derive uncertainty
relations encompassing the triple components of angular momentum, and show that
compared with the relations involving only two components, a triple constant
$2/\sqrt{3}$ often arises. Intriguingly, this constant is the same for the
position and momentum case. Experimental verification is carried out on a
single spin in diamond, and the results confirm the triple constant in a wide
range of experimental parameters.
