Maybe you're playing a hero not your style? I could win in a 1v3 against Normal AI as Unclean Beast (except in Domination, which emphasizes team size differences much more), let alone a fair match.
Maybe I'm just not able to optimize my demigod well enough, or I was playing the Unclean Beast very badly. I just tried a 1vs3 as Sedna vs Beast, Rook, and Regulus on the same map (Crucible) to see if I was right about her being nigh-unbeatable. Well the game did last an entire 50 minutes, but I still did lose. The Unclean Beast was actually the only one able to clean my clock, his damage-over-time spit + melee + ooze aura killed me extremely fast, and since the spawn was set at the default 20 seconds + 2 seconds every death (or something like that), I was getting knocked out of action for half a minute at a time near the end.
It was my first time playing Sedna and I might've done her skills really badly though, the only skill I was able to max out by level 20 were the Yetis. I was able to push the enemy back over and over so long as I didn't get ganked by the beast, but with just one demigod I wasn't able to destroy their fortresses. The Normal AI must know how to kit the beast out a lot better than I did, or I didn't maximize my healing aura enough. I've no idea.
I noticed that the enemy team's War Rank was going up much faster than mine, the funny thing is this was happening even before they started to win any ground against me. So they weren't getting more kills or wrecking towers yet but their tower was developing faster, unlocking things like the forts long before I got the same. I figured since Frogboy had mentioned their intention of setting 1 experienced player versus 3 inexperienced players in the matchmaker there'd be a way to win, but I suppose I'm just not that good at it.. Actually, I think the Normal AI is just smart enough not to make the mistakes that a team of inexperienced players would have to make in order to let an expert solo player beat them.
(edit) Just seems to me that the way the tower gets upgraded, and thus the reinforcements and fortifications also get stronger, this game isn't set up at all for the mismatched teams mentioned in the day zero post, that's the whole point of my last paragraph. I really can't think of a single RTS played in recent history that I wouldn't call 'imbalanced' though. Losing in a 1vs3 match, and losing in a 3vs3 match where the teams are as mismatched as they can get, isn't really grounds to call Demigod unbalanced.