The first gen leaf is higher, for sure, because it's almost a crossover like the Model Y (and this is nice for loading and unloading the kids, altho it hits the efficiency slightly), but I'm really not sure it's any wider. You really can fit 3 kids abreast in a Model 3.
You might need new carseats.
But I managed to get our 3 kids in the back of my first generation Nissan Leaf, which I think is slightly smaller.