Good
convertibles that harness to 40 pounds (but will usually be outgrown sooner by height--shoulders over top slots) are the Cosco Scenera, Touriva and Regal Ride and the Safety 1st Enspira.
Those ones are all made by Dorel, the maker of the 3-in-1 seats. DO NOT get a 3-in-1. You are paying way too much for a seat that won't be the last harnessed seat you'll need, and won't be the last
booster you'll need. The Scenera, Touriva, Regal Ride and Enspira give you all the same benefits of the 3-in-1 and none of the downfalls, for a cheaper price.
Of those, the Enspira is the most padded and has the tallest top slots.
Most kids will get too tall for those before 40 pounds, so when that happens you can go with a good combo seat like the Evenflo Bolero or Generations, it has tall top slots and harnesses to 47 pounds then converts to a good booster seat. But by the time you need a seat like this there will probably be something newer and better on the market.
If you can, I'd try to go with a
Sunshine Kids Radian or
Britax Marathon. Those will eliminate the need for another harnessed seat later because they have top slots as tall as the COMBO seats, and 48 pound weight limits.