3rd cold since September!

My 3 year old is literally having a week or even just a few days of being well and then catching a cold again! Currently on her 3rd cold since September. She started nursery 3 days a week in September and even before this I started giving her immune strengthening gummies and syrup as well as her usual multivitamins but it seems like they're having no impact. I'm sick of seeing her ill. When she gets a runny nose, she just cries the entire time and the nights are just as bad. I'm a single mum so have no support and when she's poorly every other week it's just draining me too. Any one else experiencing this? Any advice?