I'm tandem nursing my 6 month old and almost-3 year old. He was 1 yrs 4 months when my baby was born.
He was down to about 3x a day when babe was born--that increased dramatically when my milk came in. He wanted to nurse all the time! I think it was also because we were home so much more than usual as I was in my post-partum recovery period.
At first I let him nurse almost whenever he wanted, because there was someone else home with me to help with baby. Once we were one our own, I started to saying no more often--he would often go into a crazy fit. Honestly, the only thing that would work to calm him down was to let him watch a "movie", which for him is any video on the computer. I think for him nursing was about having a time to just chill out and calm down, and he could also do that while watching a video. Over a couple weeks we were able to get him back down to nursing only 3x a day.
For the first few months I nursed them together maybe 1-2x a week, always at nap time to get him to sleep when nothing else worked. It is a little difficult to handle logistically! A few times I felt like an exhausted, uncomfortable cow; other times I felt like a divine nourishing earth mama.

A couple months ago nursing stopped putting my son to sleep, so I no longer nurse him during the day.
Now my 3yr old only nurses at bedtime and when he wakes up in the middle of the night to pee. He just cut out his other nursing sessions over time. He is very proud that he's a big boy now!
Some ideas/advice:
Before the baby was born we talked a lot about how babies drink only mama milk, but big boys get to eat food. to try to keep him from being jealous, I tried to phrase it in a way that made him feel lucky (do babies get to eat noodles? to babies get to eat yogurt? noooo, they only have mama milk).
because all babies get is mama milk, we need to make sure the baby gets enough, etc etc
we developed comforts for him other than nursing. a cup of cows milk in his special cup, for example. there are often times now when he asks for "cup-a-milk" when he sees his lil sister nurse
I preserved a nursing time for him--bed time-- when he always gets to nurse. this is the time of day when dad is home and can take the baby so my son and I have one-on-one time. it is a special time for him.
I'm so scattered, but overall its been great. i think its been a great bonding thing for my kids... they nursed together once and fell asleep holding hands, it was amazing. but it has been difficult at times, too. being needed by TWO little ones in that way can be draining and stressful. Your 3 year old is old enough to understand some of your needs and the babies needs, to dont be afraid to draw some boundaries.