OP - it is obvious from your many, many replies that you are still justifying your thoughts about your DH going to this wedding. Why did you post if you had no intention of seeing your opinion about the wedding as being unreasonable?
However, I totally understand where you are coming from. Three children under the age of 3 is not easy and yes you BOTH agreed to having these children so close together so should equally be responsible for the childcare.
But there are a few things that, in my opinion, you are being unreasonable about. You say that he goes aways on buisness and this is okay because it isn't a wedding. That tells me you are more bothered about him going away to celebrate a wedding rather than be at home with you. Would you hold the same opinion as you do if someone offered to look after the children whilst you both went or if you all went? You say that you are bothered about 12 days of no help with childcare but this obviously happens when your DH is on buisness so what is the difference apart from the reason why your DH is away? If you manage when he is away, why can't you manage when he is away for a long weekend for a wedding? I also think it was good of him to think of asking his Mum to help you over the weekend. Granted, he should of asked you first but his heart was in the right place. If he was a pig he wouldn't have bothered. I know some may think he did this to ensure he was able to go, maybe, but he was ensuring you had help nonetheless.
Having kids shouldn't stop you from having a life of your own - and that includes YOU OP!
I will be honest and say that everything you have said points directly in the face of jealousy - and I don't blame or damn you for feeling this way. What life do you have outside of the children?
If this was me (and my DH would agree to what I am about to say) I would let him go, arrange for you and the kids to have a weekend that is easy but well planned with activities (stuff housework) and ask friends and family of your choice to help out, visits people and have junk and takeaways for lunch and dinner - make life easy for yourself. Go off routine - it won't hurt in the long term - and on his return, HE should agree to haven the kids for the same length of time whilst you do what YOU want to do.
Marriage is about give and take. Don't deny him this opportunity - weddings (hopefully) only happen once to people and I think you both need to reassess your lives. Children take up a lot off our time and we begin to neglect ourselves and our relationships with our husbands and partners. Happy parents = happy children.
I also agree that in July your kids are 4 months older. Don't stress. If you stress or loose your patience, they pick up on it, feed off it and are then the monsters from hell.
I am not trying to be unkind, although it might come across that way, but I am getting vibes from your posts that there is definite unhappiness in you and the wedding is the tip of the iceberg.
(And well done for surviving thre children under 3! I only have two and it's hard enough!)