Um, oh dear, I'm going to be the voice of doom here. Firstly, it is a stretch having three, financially. It's easier when they are young, but believe me, those insurance premiums shoot up when they get middle-aged/older. Your dad is right to be concerned about affordability. It's not just insurance. Three lots of food (and they get through it so damn quickly - do you have somewhere to store large quantities?). Larger vehicle to transport them in. There's all the non insurable vet costs like vaccinations, worming, dentals etc. Even if you are insured, you will still usually have to pay upfront, and you can guarantee that they will decide to have an accident the very month you also need new tyres, or shoes.
Also don't underestimate the practical effects. Three dogs in a house really do up the ante when it comes to housework and wear and tear. It is more expensive when you go away (three lots of kennel fees) and it's also much harder to find someone to casually dogsit. If you have one dog, it's very easy to find someone to pop in, or have them overnight, but the number of people willing to take three is VERY much smaller.
You can also think about the practicalities of walking/exercising/training them, ranging from Can you manage all three on the lead at once, to do you have the time to dedicate to ensuring they have one to one training/exercise/attention, not to mention possibly grooming. It is doable, but it eats into your time hugely.
We have four dogs (short haired), and previously had three for several years, so am coming at this from experience. I love ours to bits, but when you are a multi dog household, the time taken is considerable. Most weekdays, especially when DH is away, I basically write off the morning as that's dog time, what with exercising, feeding, grooming, training etc.
I'd say I'm happy about this as my time is flexible and I'm completely besotted with our four, but I don't underestimate that they are now a way of life rather than a pet.
Lastly, I'd be hugely wary of taking on a collie without rescue backup, especially one that may have behavioural issues (and let's face it, collies are renowned for being intelligent, sensitive and a bit bonkers, so there's a high chance that there is an issue). One walk isn't really enough (unless you are a collie specialist, in which case carry on) to make an informed assessment of behaviour in all circumstances. Can you manage all the additional training and support this dog will need, along with the needs of the other two?
If you can, and are still determined, that' brilliant, but it's important you understand some of the reality rather than just the nice bits.
Good luck.