Also, if you're going to have totally rediculous encounters like this, you need a PROPORTIONATE REWARD. There is nothing of value in this entire dungeon. If you're going to make a level 3 party fight a boss with 125hp that can wipe your entire group out in 1 round, and has potentially 15 or so allies, you should offer a big, proportionate reward. This is DM'ing 101.
Larian nor anyone "made" anyone fight the Phase Spider Matriarch. You CHOOSE to. Its optional 100 percent. Complaining about an Optional fight being too difficult is silly as you can come back to it at higher level or you can just not fight it at all.
And btw some people consider a challenging fight and the experience your characters get as plenty reward. Not to mention you get plenty of things worth gold and if you have a poison user a great poison item and a spoiler item is in there. So.....there is quite a bit of reward in there even if it didn't fill your reward bowl how you like it.
Also if you are level 3 and have Karlach as Wild Heart with Bear totem then.....there is no way you get wiped. Plus if you keep people spread out they can't all get hit at the same time. If you play a challenging fight poorly you will get wiped. Welcome to tactical turn based RPG combat, and fans of the genre wouldn't have it any other way. You can split the party up before initiating the fight even to avoid it even being a remote possibility. Please don't act like the game is bad or in the wrong just because a fight goes poorly when you aren't playing tactically. Think tactically and use your environment to your advantage. A simple mobile flourish as a bard will send the matriarch flying into the abyss. A firebolt or acid attack on the web bridge will break it causing creatures on it to take fall damage and go prone. Lots you can do here.