I nursed my three for a few months after ds3 was born. By that time, ds1 was down to one before bedtime nursing only, and he knew that we would be ending when he turned 5. With my younger two, baby got first dibs and then ds2 was able to nurse. On occasion, when they really needed it, I would nurse the youngest two at the same time, but I've never really liked doing that.
The only way I have personally managed to tri-andem nurse was to have some limits and nursing ettiquette rules. It has worked out pretty well, because my younger ones have learned the rules from watching the older ones. I will end a nursing session with an older one with a 10-count. The first time I did this with my youngest (once he was a toddler), it was at night and I really needed to get up to go to the bathroom. He was half asleep, and I counted to 10 in desperation... and was shocked when he unlatched himself, rolled over, and fell back to sleep!
I did nightwean ds2 before ds3 was born. I did not nightwean ds1 before ds2 was born. It hadn't seemed necessary since most nights he slept through on his own... until his baby brother was born, that is.
I am hoping to be nursing three again when new baby arrives in October. I wouldn't mind if ds2 weaned himself before then, but that is very unlikely. I am kind of worried ds3 will wean, though, and I'm definitely not ready for that. That's another post, though.