i coming up on 5 yrs of continuous tandem nursing (that will be in august) of which the last 2 have been triandem. i haven't had a child completey wean yet, but my oldest is pretty close.
1 & 2 16mos
2 & 3 31mos
3 & 4 expecting it to be 27mos
now, i'm not a matyr and i haven't been suffering. i truely have enjoyed my experience in tandem nursing. too me, it's just like nursing only one, just + one. that's why i have 2 breasts.
it's not always great...i'm not too fond of nursing 2 at night (which i did for over 2yrs) and that has issues too like is one keeping the other going longer especially since #3 (who's not been tandemed at night) has nightweaned on her own by age 2 (she's still going back and forth on this, but currently she's not nursing at night her decision). but after 2 at night, i never did complain again about 1 at night. so it looks like it will still just be one after this one is born. hopefully.
and nursing while sick or pregnant is tough too. i have noticed (in my experience) that the closer they are in age, the better nursing through a pregnancy is as far as keeping the supply going.
of course, the bigger the age gap, the more you can reason with the oldest especially when you can't possible nurse 3 at the same time. usually i could get the oldest to hold off for a few minutes. and then alternate him with the 2nd oldest.
i've dealt with bottlenecks (when 3 want to nurse at once), kids fighting over the same side, kids getting upset that they are touching their sibling, etc. but we've worked through those.
i think pulling the nipple out of siblings mouth is funny, and all mine have done it. it's neat to see the older one get the nipple pulled out his mouth by the youngest.
i've seen the good stuff too. i've seen tenderness between the siblings, eye contact between them and me, we've joked and tickled, i've had two fall asleep (probably fighting previously because they were tired). i've watched my youngest when she could finally climb onto the couch, drag her oldest brother off the breast and then take over, and the snuggling with a baby in my belly. i've been nursed! (while one is waiting sometimes i get a back scratch/rub by the one in waiting, so i call that nursing mama)
while nursing two together, i've knitted, read books, read books to them, conferenced called my boss, worked on the computer, etc.
i've even had my oldest latch on, get off, and say "i'm going to go get you a glass of water, you need it".
tandem has been a valuable in my roll as a mom. i'd hate to be without it. it's not perfect, nothing in life is. and i think you can get burned out in any aspect of your life from time to time. and excepting that your not always going to have your good days, makes you appreciate the good days more. i would do it exactly the same way again if i had too!
and there is no magic age gap. they all have their benefits.