I tried to select a random armor_stand by using the @r selector. Until 1.12.2 this has been possible using this:

// 3 times this command at various locations:

summon armour_stand X Y Z {custom name:"random"} 

//to chose one of them randomly:

execute @r[type=armor_stand,name=random]   ~ setblock   ~ stone

P.S.: I know that the syntax has been changed with the new snapshots for 1.13, but even by converting the command with the new syntax it doesn't work:

execute at @r[type=armour_stand,name=random] run setblock   ~ stone