at what age/ height/ weight can LOs be in a backless booster with just a seatbelt? I'm wondering if we can use DD's convertible car seat for DS when he's outgrown his infant carrier, which he just about has. So at age 3, about 36" 30 or so lbs would we be able to use a backless booster?

Teterboro 5K 7/16/11 23:22
Tenafly 5K 6/5/11 26:48 1st in age group and stroller division
Teterboro 5K 7/17/10 24:42
Lincoln Tunnel 5K 4/25/10 28:18 4 1/2 weeks pp
Teterboro Airport 5K 7/18/09 22:35 3rd place age group 4 1/2 weeks pregnant
Long Branch 1/2 5/3/09 1:51:07
Lincoln Tunnel 5K 4/26/09 22:22
NJEA 5K 11/7/08 22:30 2nd place age group
Westchester 1/2 10/12/08 1:50:16
Teterboro Airport 5K 7/19/08 23:43
Long Branch 1/2 5/4/08 1:54:18
Giant Stadium 5K 4/26/08 error in timing
Hackensack 5K 10/14/07 23:55 1st place in age group
Re: backless booster seat
No way. A five point harness is recomended until they are 40 pounds.
Ditto this. The minimum age for a booster is 4, although a better indication of readiness is their ability to sit nicely 100% of the time. No leaning over, no playing with the seatbelt, etc. Once they are at least 4 and mature enough to sit nicely they can be moved into a high backed booster. We moved DD from the 5-point harness to a high backed booster when she was 6.
Ditto the pps. It not okay to put a three year old in a backless booster, regardless if they fit the physical criteria on the box. There have been kids in the news lately the have been killed or seriously injured at this age because of misuse ofhe seat, child notsitting correctly, or despite fitting the criteria for the seat, are short torsoed and get a bad seat belt fit and torpedo out of the seat. These are typically accidents where other passengers have minimal injuries. Heck unless your three year old is above the 100%ile for height, they really should be rear facing, or at the very least harnessed forward facing. You can google this for yourself if you like, or ask a tech on car-seat.org, you don't have to listen to an Internet stranger.
If you need a new seat, there are lots of appropriate seats for low budget that are perfectly safe.
All this. The 40 lb minimum is also important to make sure the adult seat belt works correctly.
We moved DD1 to a high back booster at 5 years, and she'll be in that until she outgrows it. Then we'll move her to a backless booster. She's 6.5 yrs, 53 lbs, and 51" right now.
Charlotte Ella 07.16.10
Emmeline Grace 03.27.13
We are similar to this, my 7 1/2 year old is still in a 5 pt harness (unless he goes with friends, and then we let him use a booster). My 5 1/2 and 4 1/2 are still in 5 pt harnesses, and my 3 year old (29 pounds and 37 inches) and 18 month old are still RFing, and will be for quite some time.
Each "step up" in the car seat world mean less protection from a car accident- no need to rush!