... and they are all the same size, IME. They are designed for children who do not yet sit up, so most babies outgrow them by about 9 months, if not sooner. On all US carriers a three year old is required to have his/her own seat on the plane, that is, you must buy the child a ticket. If you plan to use a carseat for the 3 yo, don't take the bulkhead seats; the carseat probably will not fit in that row, because of the fixed armrests.
If you are the only adult going you can't easily use two carseats, as the FAA rules say that carseats must be secured in the window position-seats; they -might- let you put two next to each other in a 3-across row with you on the aisle, but I kind of doubt it. If there are 2 adults and you are using 2 carseats, then you should get two rows of two-across window/aisle seats, if possible, a block of 4. That way the older child will only be kicking the baby's seat, and you won't have to keep admonishing her about it. (The baby won't notice, but another grownup would.)