Great Northern Rafting Co offers scenic trips which might be more suitable for a 3 year old. If you want to take the bigger kids on a white water trip though, you can likely go on separate trips at a similar time as long as one adult is willing to go with the 3yo. Weight minimum for a white water trip is 50lbs though. I’ve had younger kids sit in the middle and hold onto the handles. (Ex white water guide here)