For 3 minions it is actually 1/9, since the first minion hit can be any of them. Same argument (i.e., it will happen multiple times per day), just more so 
One question I have: Can devolve missile hit a minion which is already at cost 0? That could have an influence then, if you have 1 big minion and 2 with cost 0, it would only have one target to hit, really.
I tried to find streams of Devolving Missile to look at the situation myself, but could not find one easily. So if one of you has links to such videos (please specify at what time(s) the card is played), please share them. Don’t forget to follow the guide:
https://us.forums.blizzard.com/en/hearthstone/t/how-to-share-links-to-screenshots-videos-and-replays/2242