The child is newly 3 yrs, 39in and 39lbs, not sure of the car. Below are your recommendations, which out of those would most likely be the easiest install, generally fit in most cars.
Combinations/Harnessed Boosters (forward facing with harness or as a booster without harness)
Evenflo Securekid ($$): 22-65 pounds harnessed; Booster to 110 pounds. Reasonable price, good for cars with LATCH.
Graco Nautilus Elite ($$$) or Argos ($$$): 20-65/80 pounds harnessed; Booster to 100 pounds. Tall seats available in many colors, can also be used as backless.
Recaro Performance Sport ($$$$): 20-90 pounds harnessed; booster to 120 pounds. Tall seat with unusual (but not difficult) seatbelt installation, very deep sides.
Britax Frontier ClickTight G1.1 ($$$$): 25 lb - 90 pounds harnessed; Booster to 120 pounds. Highest harness of any seat currently available, nearly foolproof seatbelt installation.
Combinations/Harnessed Boosters (forward facing with harness or as a booster without harness)
Evenflo Securekid ($$): 22-65 pounds harnessed; Booster to 110 pounds. Reasonable price, good for cars with LATCH.
Graco Nautilus Elite ($$$) or Argos ($$$): 20-65/80 pounds harnessed; Booster to 100 pounds. Tall seats available in many colors, can also be used as backless.
Recaro Performance Sport ($$$$): 20-90 pounds harnessed; booster to 120 pounds. Tall seat with unusual (but not difficult) seatbelt installation, very deep sides.
Britax Frontier ClickTight G1.1 ($$$$): 25 lb - 90 pounds harnessed; Booster to 120 pounds. Highest harness of any seat currently available, nearly foolproof seatbelt installation.