IMO (I have ADHD and 2/3 DC diagnosed) it's inappropriate for preschool staff to be suggesting ADHD at age 3 - it's basically impossible to tease it out from the active/intense side of normal at that age.
They could say things like "she has a lot of energy" or "she seems to display some sensory seeking behaviours" or "she's extremely curious" but I don't think it's unusual for a 3yo to struggle to pay attention to something they aren't interested in - is it?
I appreciate my views may be skewed because the majority of my experience of small children is clearly ADHD-focused. But I did also used to run classes for children and 3yos' attention span in general is like trying to herd cats. They're all like that. Luckily it's fairly predictable what will interest the vast majority of them so it's not that difficult to herd them (I suppose they are more like puppies in this sense.) I wonder if she has unusually intense interests and this is why they are finding it hard to interest her in the tasks they are trying to?
IME the biggest issues at the preschool age for children with ADHD are emotional regulation and the sensory issues because some have a really huge, insatiable need to either move constantly, be making constant noise or sometimes it's touching things/chewing things/trying to constantly lie on people or burrow their head/feet into you (which is usually a sign they are seeking deep pressure). You can address these all directly - there are loads of great emotional regulation programs for preschool aged children which typically benefit all the children, not only those who struggle with emotional regulation, sensory seeking can be addressed by providing appropriate outlets for those needs. Following instructions can also be an issue - I would suggest that they give her some "jobs" to do - I would also do this at home as it is hugely helpful if you can get them used to it early on. This can help her to feel useful/responsible/grown up and get praise and recognition for the tasks that they want her to do anyway, and it helps explain why those rules are in place. Oh and potty training/eating can often be issues, related to sensory sensitivities.
If it is ADHD then none of that will prevent it from continuing to be apparent into later ages/stages, but it can help strengthen areas of skill which are often difficult, and if it's more an "intense end of normal" thing then it will help that phase to pass a bit more smoothly.
But it could be any number of things at this age. She could be highly intelligent and bored. She could just have some sensory issues which will sort themselves out as she gets a bit older. She could be struggling to hear.
I think it's unlikely that she will be pigeonholed simply because I think it is incredibly unlikely that you will get an NHS paediatrician to diagnose ADHD at age 3 unless it is one of the absolutely extreme cases which you hear of very rarely for example a child of this age (which I have heard although online, so take with a pinch of salt) who is routinely escaping the house and taking walks down a country lane in the middle of the night, or taking the car keys and trying to drive the family car.
But OTOH the waiting lists are years long - the cynic in me says join it if it's offered, because if you are having horrendous problems at age 5/6 (which, touch wood, I hope you won't!) you don't want to be at the beginning of a 3 year wait then for answers. And OTOH if you get past this point and she does better with a bit more maturity and patience then you can always cancel the appointment.
I think it would be helpful if there was a category where they could say there are some flags here which might later point to ADHD but it's too soon to really say, but in this group of children who have this sort of "possible ADHD" profile, these are the things which help. (Sensory supports, emotional regulation support, age-appropriate responsibilities, visual timetables for routines or to show what's happening next, specific directed praise, etc)