This is tricky and may need some experimentation depending on the seats you have. I've got an arsenal of 6 different car seats that I've used in 3 different cars, from really big SUV to tiny sports car, and then the RAV.
Hopefully the seats you have/will be getting will be narrower models as many normal models just flat won't work. So many child seats and infant carriers are waaay wide due to all the side protection one-upmanship between brands.
Physically, the bench is wide enough. The problem, as mentioned already, is the sub optimal latch and belt positioning for 3 across. Great for 2, as the latch centers the child seats in each bucket. But it's hard to take advantage of the full bench width that way. Three across needs the seats at the sides all the way against the door. The only way to do that is to use the standard lap/shoulder belts. Then use the center-most latches for the middle seat. It's not ideal and no manuals will recommend it, but it can definitely work well with all seats tightly secured.
Another trick is to stagger the seats in height. That allows the side bolsters/arm rests of the child seats to kinda stack over one another. For example, some car seats (especially convertibles) have a thicker base that elevates the car seat. You might be able to use the infant carriers without snap in bases along each side of the convertible for your 21mth old at the center position.
Good luck!