I agree with the previous poster, at just 3yo, your daughter really needs to stay harnessed for another couple of years. I understand that she is very large for her age. So is my daughter. She was 38 inches tall and 35 pounds at 2.5 years old when she outgrew the Alpha Omega harness by height (shoulders over top slots, second-to-top headrest setting). She was 41 inches and 40 pounds at 3 years, 4 months old.
Back when my daughter hit 40 pounds at 3 years old, there were
very few options to harness her, either Britax 65 pound max weight limit convertibles, the Britax Husky (older version of the current 80 pound max weight Regent), or the now-discontinued 50 pound max harnessed weight limit Nania Airway. None of the 65# Britax convertibles made sense: she was too big to utilize the rear-facing function of those seats, and so tall and long-torsoed already that she needed the top slots, with little growing room (indeed; she was too tall for Britax 65# seats by 4.5 years old). We ended up with both a Nania Airway (for lesser-used vehicles) and a Britax Husky/Regent (for the primary vehicle). The Husky/Regent lasted my daughter until she was 7.5, about 54 inches and 70 pounds.
The Britax Frontier and Graco Nautilus are also good options to consider, choices that weren't available when my daughter was outgrowing standard 40 pound harnessed weight limit seats. She would have fit the harnesses of those seats until she was 6. In my honest opinion, the Frontier's belt positioning booster mode blows the Nautilus out of the water, so if I were in the market today for a seat for a 42", 42# child, the Frontier would most likely be my #1 choice, since it should last even the biggest of kids to age 5-6 in the harness, and it makes a much better booster than the Nautilus.
The Britax Regent is also still a good choice, though it is wider and bulkier than the Nautilus and Frontier, and does not become a belt positioning booster, it will harness longer (1-2 years longer for most kids).
How is she fitting the Marathons? Where are her shoulders in relation to the top slots? She will outgrow those seats by height when her shoulders are over the top harness slots. At her height, if she is long-torsoed, that could be anytime. If she is average or shorter in the torso, you could still have 1-2 years left.
How much time does she spend in the other two vehicles? My first inclination would be to move the Marathons to the two lesser-used vehicles, and get a Frontier, Nautilus, or Regent for the primary vehicle.
Having said all of that, my daughter did begin booster-training (meaning, less than 5 minutes per trip, with no other children in the car to distract her from proper booster behavior, gradually working up to longer trips when she proved she could handle it) in her dad's vehicle at age 4. I did not trust her in a booster any longer than 10-15 minutes until late 5, and she didn't move to a booster full-time until age 6. We used a Regent part-time (longer road trips) until after her 7th birthday. I would delay booster-training for at least another year, and even then, at 4.5, I would evaluate whether or not booster-training is a necessity yet. How often does she ride in these other vehicles? Do the seats stay in these vehicles, or are they uninstalled? Are there other children in these vehicles with her? Will there be long road trips in these vehicles? All of those questions, and I'm sure a million more
should play into the decision to booster-train.