Indeed, too few women for the men very often leads to instability.
I don't know if you ever followed what happened to the men on the HMS Bounty, led by Christian Fletcher in a mutiny against their captain. Most hid on Pitcairn island along with some Polynesian men and women. When they were rediscovered only one of the Bounty crew survived. All the rest were dead, having killed each other over various disputes, prominent among them fights over the women, who were much fewer in number.
It is speculated that part of the reason why, for example, in the Indo-European culture, where powerful chiefs had more than one wife, young men were encouraged to go raiding for land and women. It breeds turmoil either within or without.
The same shows up in polygamous cults here in the U.S. where the elite men just expel their surplus sons when they reach about 17 or 18.
Not that some cultures don't practice polyandry, but the most common form it takes is fraternal polyandry, where two brothers marry the same woman, i.e. Tibet etc.
It is mainly practiced in poor families with small landholdings who can't divide the property among the offspring of separate fathers or they'll all face starvation. So they keep their property big by having equal access to the same woman. The property still stays in the blood line, so to speak.
There's a hint of that kind of thinking among the Jewish ultra-orthodox, if I'm not mistaken. When a man dies, his brother is to marry his brother's widow. It's my understanding that nowadays it only applies to a brother who is unmarried at the time. It's also my understanding that the woman has to be released by the brother if she wishes to marry someone else. It's to continue the bloodline, or perhaps originally also to make sure the widow wouldn't descend into poverty. To clarify, I've never studied the issue, but was told this by my husbands' partner, who is Orthodox.
At any rate, it's all speculation when we're talking about practices in pre-history. One elite man taking multiple wives has been a common practice in many cultures, but three women for seven men? Why would that even occur? Female and male birth rates skew female if anything, and men get killed in battles.