Hi H.....
Congratulations. But I know exactly how you are feeling, although I didn't have three kids before my twins. I just had twins (boy/girl) this past June. It was a TOTAL shock to us. There are days, even now, where we think "how did we end up with twins?". While we feel doubly blessed it is a totally different ballgame. They say God only can give you what you can handle and I joke that God must have been drunk that day for Him to think I can handle twins. ;)
They say it's easier to have twins before other children but I just think it depends on your situation. Luckily your 6 year old can help out a bit and your 3 year old will be nearing completion of potty training.
Be sure to check out (do a google or I can send you some stuff too) for a lot of twin discounts/freebies. It does help.
Do you plan to nurse? If so, the twin e-z pillow (available on twinstuff.com) was KEY for me.
Twin pregnancies are considered higher-risk but that doesn't mean you can't have a healthy and full-term pregnancy and even two vaginal deliveries. My biggest advice... make sure your hospital has a good NICU JUST in case. You'll also want to ask your ob/gyn what your dietary needs should be - they may want you to eat more sooner, take iron supplements, etc. He/she will know best for you. For me personally, I started having pre-term contractions at 23 weeks. And I was put on bedrest. I ended up delivering at 34 weeks. The kids were in the NICU (3 dif. hospitals for 15 days) mainly because I don't think my deliverying hospital knew what they were doing. I had no epidural and delivered both vaginally. I kinda miss out on the whole 2nd semester "have some energy back, not as tired" feeling - pretty much went from like 1st to 3rd. :)
I found www.twinstuff.com forums INVALUABLE for me. I STILL go on it at least once a week and post my panics. :) It's an EXCELLENT resource. You may also want to find out your local Mother of Twins Club Chapter. If you live in the city, Pamela Chay out of NW runs an excellent group. But there are many others. I haven't taken advantage of those since the one out by me (we since moved to sw burbs) isn't very good.
I noticed you said DH can be lazy. ;) He'll need to snap out of that really soon with twins on the way. ;) If anyone can help you during the first few weeks that would be wonderful. We didn't have any help. While I am proud to say I got through it on our own (although months 0 - 5 are a TOTAL blur) some help (even if it was just for me to get out of the house for an hour or sleep an hour) would have been great!
Feel free to email me too - just send me a private message.
Congratulations again. Try your hardest to breathe in and out. :)