I wonder if it's worth sticking it out until your son is a bit older because a lot of couples feel like ships passing in the night and that all they talk about is the kids when they are young - especially with three of them!
I've only really skimmed the responses so I don't know of anyone has already said this but the thing that jumps put to me is this.
You say your libido is very low and has been from the start. Yet your husband seems content enough and isn't pestering you for more sex. I agree with the poster who said that without desiring dex, you're not going to feel the butterflies - it's fancying someone and desiring to have sex with someone that creates those butterflies and excitement.
You also say you miss the excitement of a new relationship and all the firsts.
My thoughts would be this.
If it's hard for you to find time to date your own husband or spend time with him because of the children and lack of childcare. How would that change if you split up? People talk about sharing care 50/50 but I've only met a couple of people who did this in reality. In most cases, mum has the kids 26 days/nights a month and dad has them for 4. So your opportunity for meeting someone else would be limited at the moment anyway. Especially if you're a bit of an early bird who isn't going to want to do stuff in the evenings anyway.
If you did meet someone else, they would likely not be content with sex every 6-12 weeks and so would either pester you for more or be seeking it elsewhere behind your back. Neither of which is particularly desirable! If your libido has always been low, is that related to attraction levels to your husband or is it just the way you're wired? Because if its how you've always been, it's unlikely to increase with someone else and you could end up chasing those firsts with someone new for evermore. There's nothing wrong with that. I have a friend who is nearly 60. She's very attractive and in really good shape and looks 10 years younger (genuinely!) She has no interest in a relationship because she becomes bored easily with them. She has short flings when she fancies it because she has no children and is entirely financially independent. She experiences those firsts often! But she isn't looking for anything more and it's not really a lifestyle that's easy to choose with 3 young children.
The reality of dating nowadays is that, if you're not able to go put do hobbies/spend times pursuing interests on your own etc where you might meet someone in real life, you're left with online dating. This is not the world of opportunity you imagine it to be before you've done it! Believe me!
At the moment, you think the grass will be greener. The best response to this I ever heard was that the grass is greenest where it is watered.
The responses you've had seem to be focused on refinding the spark with your husband and I think that is very wise. You love your husband. You're just a bit flat about the whole thing at the moment. You're not repulsed by him. You're not upset by his behaviours. You don't talk about having low self esteem or being criticised by him. That would likely all change if you pursued excitement and firsts by dating elsewhere. A lot of men really aren't very nice...
Do you have hobbies? Interests outside the home? Interests you shared together before the children came along? Can you take these up again? Find something you can do together? Something else you could talk about instead of the children and work?