Flying long haul at any age with triplets is going to be challenging. More so at 18 months than 24 months.
Avoid flying long haul with children aged between 12 - 36 months, especially if you have more than one to contend with. (One is easy. Completely irrelevant to three... especially triplets!)
But that's not realistic, is it? So you have to suck it up at some point and just do it.
Tips to help: Don't eat or drink too much because you won't have time to go the the bathroom at all.
Don't drug. I was advised NOT to drug my kids (flights are too long, kids can't possibly sleep for all that time and when they wake after an 'assisted' sleep, they tend to be grumpier than when they wake from a regular sleep) and I'm glad I didn't because my three are all sensitive to antihistamines and go slightly hyper when given a dose. (Which would NOT have been fun on a plane.)
Try and get seated away from the 'nursery' (airlines seem to bunch all the kids into one area so that if one wakes up and cries, they all wake up and cry. Great fun.)
Split your kids up if you can. One for one parent (the lucky one!) two for the other parent.
Use carseats if you can afford it. Most airlines like kids to be buckled in turbulance and it's easier to have them buckled in all the time. Easier for meals etc. too.
Good luck.