If she won't rear-face, you're wasting half of your money by buying a convertible (Size4Me, Marathon, etc). Better to get a combination harness-to-booster seat. (they'll last longer harnessed than the convertibles too, even if you want to get a dedicated booster later).
Look at Graco Nautilus, Evenflo Maestro, Britax Frontier/Pioneer/Pinnacle.
For a 30 lb 3 year old the Frontier or Pinnacle are probably overkill, unless you want to keep her harnessed to 12 or so.
The Pioneer costs less and is comparable to the Nautilus (currently $160 - $190 on Amazon depending on color/pattern).
I like the Nautilus a lot for the price and what you get (but I bought mine when the Pioneer was not sold yet, so maybe I would have gone with that). Frontier costs quite a bit more, but does last longer (but the extra harness height / weight capacity are more than most kids will need - good for very large kids, or children w/ SN that may need to be harnessed longer than typical). My 7 yr old (45" and ~42 lbs, closer to the size of most 5 yr olds) still fits in the Nautilus with a close to 2" to grow (torso height) before he outgrows the harness.
I don't have any personal experience with the Pioneer or Maestro. Maestro is shorter than the Nautilus, and the booster (IIRC) is outgrown at the same time as the harness, but it costs quite a bit less. Good option if you know you'll get a separate dedicated high-back booster later anyway.