M.,
I believe the toddler bed is really about a bed being functional for your child. Some people skip the toddler bed and have a twin bed on the floor or a low bed. That way the child can climb on to it safely and easily. My point is, you might want to consider getting matching twin beds and just putting the mattress on the ground so it's low and easy to get on to. A toddler bed is fine too, but I think it is only a short time experience. Your three year old should be getting to where he can easily get off and on the bed without much difficulty. Also, don't forget the guard rails that keep them from falling off.
Your one year is probally still moving all around hat night. so, make sure the bed is low and you have protection from him falling off. Many people keep their child in a crib until they are three and that sounds like what you have done with your first child. It's all about being ready and where your child is at on an idividual basis. It's nice your one year old can watch his older brother and learn from him all the bed time routines he needs to know. It's really up to you what feels like the right situation. I am sure there is probally a certified standard for everything and I think it's logic that makes the most sense.
Best wishes to you and your family. And what an exciting step for your three year old to be getting a big boy bed soon!
G.