You have gotten some good advice but some of it does not apply to international travel. I have a 2 year old that we have done LONG haul international flights with since she was 3 months old. We currently live in the Middle East and travel back to the US about 3 times a year and then have also vacationed in Sweden and England.
1. Agreed, there is a fold down changing table in the restroom but NOT every restroom - usually one in the front of the plane for Business and First class bathrooms and one at the very back for Coach class. Ask when you first board the plane/s where the fold down changing table is located.
2. You can gate check the stroller at no extra cost, however, you mention a few flight changes and depending on where you fly into you may not have access to your stroller when you land. For domestic flights (within the US) it shouldn't be an issue but if you stop somewhere internationally before arriving at your final destination then you may be stuck without your stroller for the next flight. (I hope that this makes sense). My advice on that point would be to plan on NOT having it (sometimes the flight crew isn't aware and will tell you it will be available when it might not be - which has been our experience more than once)! Have your carry-ons and the kids ready to get off the plane then once you are in the terminal - have one parent watch the kids and bags while the other one goes to find a luggage cart which usually have a place for ONE kid to ride. Some airlines and some airports have strollers that you can borrow but I wouldn't count on it! Also, parents traveling with small children can request a ride (much like older travelers or people who have trouble walking the entire distance of airports) - don't be afraid to ask for this! Don't try to "tough it out" - especially since you have several flight changes - take advantage of this service and don't make yourself miserable!
3. Agreed, request the bulkhead seats and a bassinet for your 12 month old - your baby may weigh too much for the hanging bassinet but they still might bring it and let you put it on the floor to that you have a place to "contain" the little one! My daughter started scaring me in the bassinet when she was able to sit up and thought it was HILARIOUS to climb out of the bassinet and jump on me while I was sleeping (I was on a 16 hour flight alone without my husband on that trip)! I was scared to fall back asleep!
Since you will be on long flights and have layovers - be sure to bring enough diapers - we plan on 1 diaper for every 2 hours even though she usually doesn't go through that many - you never know when a kid will get sick or go through more diapers especially with trying new foods, etc., food and snacks that your kids like (they may not like what they are serving on the plane)! Bring a change of clothes for each kid and you too - kids never get sick just on themselves! Also bring some meds just in case they get sick or run a fever (the plane doesn't stock any and as long as it is meds for kids in a ziplock you will be allowed to bring it through security)! I didn't see you mention this but SO many people think it is a good idea that I want to say DO NOT give your children Benadryl before flying (especially if they have never had it before)! Some children have an opposite reaction and HYPER out instead of falling asleep! You don't need to drug your children in order to have a pleasant traveling experience! Especially since you mention several flight changes, you don't want the kids passed out when you are trying to change planes and go through immigration, etc.
What we have started doing to entertain our daughter on flights is to wrap a "present" or two to let her open when she gets bored with the books and toys that we have brought. It doesn't have to be anything NEW - sometimes I just wrap in tissue paper a toy that she likes but doesn't know that we brought on the plane! When she was younger, she just liked wrapping and unwrapping the stuff but now she wants to play with the actual items. The best bet for your 3 year old is probably a new toy that will keep him/her busy! My most recent treat for my daughter was a Dora the Explorer magnet and book set - got it at Borders for $13 - they had a few of the popular cartoons (Sponge Bob, Spider Man, etc.) - just an idea and she is still playing with it!
Good luck on your travels - remember to relax and enjoy! If you have any other questions, I am happy to answer them.
blessings,
Stacy