You will if you go anywhere like the mall, zoo, amusement park but not for errands. A sit n stand IMO works okay but a 3 year old still wont sit down and understand not to just get up and walk off and they often need a place to nap!
we have a side-by-side and an incline and I love both for different reasons but if I had to suggest one for a 3 year old and newborn it would be something that is a single which can convert to a double (we have city select). We used the car seat attachment, sometimes just a single, sometimes double, and we have a glider board so she can ride on the back for a quick trip as a single.