A bit of a dumb question but I'm not sure - I understand that you can use a high chair when a baby can sit up but as there seems to be a long time between sitting in a bumbo/propped up and really sitting up alone without using hands, when can a baby sit in a high chair? In case it makes any difference to the answer, we're going to go for a Trip Trap (sp??) with the baby padding.
I put both my ds's in a high chair as soon as I weaned them onto solids (6 months). They couldn't sit up entirely on their own until 9 months, but they were absolutely fine/ secure enough in the highchair (which 'reclined' slightly, which helped).
What I mean is, I probably would have started using the highchair at that point whether she could sit up unaided or not. As long as she is strapped in securely (you will need to buy a separate harness for the Tripp Trapp) and use the seat cushion I would have thought she would be okay.
my dd was able to sit in her high chair at 3.5 months, she leaned against the back but she wasn't uncomfortable or anything. think it depends alot on the baby, give your LO a try in it and see if she can manage, only way to know for sure
My ds was in a basic wooden highchair at 3 months too as he absolutely hated his bouncy chair - didn't like the angle and I also think he wasn't very happy to sit low down and just observe feet passing by when I was trying to get chores done!!! I thought that if he was on a level with me in the kitchen it would be better for him. I just sort of propped him up with a book under the seat padding and stuffed muslins down the side to prevent too much loll!!! He loved it...
I still prop my ds with a cushion behind him in the highchair at 10 months, because although he can sit up perfectly well, he wriggles about when he's strapped in the highchair and ends up a bit slumped!