Re: baby #3
We haven't used a bucket seat for the last two; when ds2 was born, dd was 2, a runner and as stubborn as the day is long. When ds2 was a noob, I often tandem wore dd on my back in an oh snap, and then got ds2 situated in a ringsling or beco in a front carry -- I am a man, though short, and I found this easy, but it doesn't sound like that's an option for you. Of course, this was in the cold snowy winter. Were it any other time of year, I'd grab a cart, load the older child in the seat, and wear the noob. I was also comfortable allowing the two year old to walk beacause I knew I had the strength to do a front carry and pick up dd if she didn't want to hold hands without smushing ds2. Ds1 was just 5 at the time, so he could follow directions.
Personally, I'd go the convertible seat route, and load the 2 year old into the cart before loading the Nb into the carrier.
SAHD to J 12.27.05, M 12.27.08, and A 2.15.11.