In my experience there is enough range in the length of thread in the nipple to use the spares on either side of the rear wheel.

Spares are typically the size of the drive side rear where 95% of broken spokes occur.

Still, when I needed to replace a non drive side spoke due to a piece of road debris getting into the spokes, the spare worked fine even though there were threads showing due to the fact that it was not the optimum length.

Front wheel spokes are seldom a problem, but on my bike they are pretty close to the same length as drive side rear ones.

Bottom line... One size will get me going as a replacement for any of the three sizes in a pinch. I am inclined to put the correct size in later when it is convenient, but would be in no rush to do so.