Hello
@CandyHasCakes -
How long have you had the 2 youngest cats? Did you get them separately or together? Has your oldest cat been with you longest?
Did you make the introductions slowly, keeping the older cat apart from the younger cats, and allowing a good period of time for the cats to get to know each other in "safe mode" i.e. through a screen door?
There has been a bad start to the relationship, and now the poor older cat is being bullied a lot by the other two. The only way to change this sad situation is by separating the cats into 2 social groups (the 2 younger cats in one group, and the older cat in a 'group' on her own.and starting the introductions over again. This time you'd need to be prepared to take several months over it.
The two alternatives would be either to set up your home so the 2 younger cats are separated permanently from the older cat. (i,e, divide your home in 2 with a fixed divider.)
Or else rehome the cat you acquired last. Or rehome the cat who is causing the most trouble for your older cat.
Whatever you decide to do, things can't be left as they are, as it is very stressful for a cat to have to live with cats they don't get along with, or who are bullying them. There is a risk that older cat will become unwell with a stress-related illness.
Have you already tried Pet Remedy plug-ins, Feliway plug-ins, or a course of the calming supplement Zylkene for all 3 cats (add the powder from the capsules to their food).?
If not then I would buy the plug-ins and the Zylkene supplement, and then immediately re-start the introductions as I described above, using a screen door. Take your time, do not rush things.
EDIT : are all 3 cats neutered? This is very important if you want them to get along.