for your 3.5YO's size, if the Nautilus or Frontier are out of your budget range you could probably get a few years good usage out of an Evenflo Maestro (about $80 retail - harnesses to 50 lbs. and then can be used as a belt-positioning booster. Doesn't adjust as tall as the Graco Nautilus or Britax Frontier, but by the time she outgrows it as a booster you could get a good basic booster like a Graco Turbo Booster for $50 or less)
The Maestro, Graco Nautilus, and Britax Frontier are the combination seats (forward facing harnessed seat that converts to belt positioning booster when harness is outgrown) most frequently recommended by certified child passenger safety techs for their high height and weight limits for the 5 point harness and for positioning the lap and shoulder belt well in booster mode. Most other combo seats are either outgrown too quickly as harnessed seats, position the seat belt poorly as boosters, or both.