Your family is correct in that disagreements about money will most likely cause issues in the long run. So you have to make sure that you're all on the same page right from the beginning. This means talking things through and find compromises that could work for everyone.
I haven't been in the same situation as you but I do have experience planning to do stuff with a good friend that has a disease. Her situation means that she has a lot of medical costs and only basic living income about 1/3d of my own. It's obvious to everyone that she won't be able to afford certain things that would be trivial for the rest of our friends.
My initial reaction (similar to your friends) was that I wouldn't mind paying for things like ordering pizza or going out to an amusement park. This was shut down when she said she wanted to be normal like everyone else and pay for her own things like you would expect normally.
This looks to be really similar to your concerns of feeling like a freeloader when your pay-happy friends would buy the things instead.
option everyone cheap but happy
In my case it made me realise what was most important to my friends. In most cases this meant that we would have more fun staying at home (often her place because transportation is difficult for her) and we would order food where everyone pays for their own.
It's possible that once you make it clear to your friends that you really can't afford the more expensive stuff and you feel like a freeloader when they pay for everything, that they might tone down their expenses as well.
Since they will be living with you I doubt this is the case though so let's look at other possible compromises instead.
option usage fee
Maybe you can agree that they will buy certain expensive things (like furniture) and you pay a certain amount you feel comfortable with to use it. This achieves a couple of things:
- Your friends have their comfy/fancy furniture and/or other stuff of their own choice
- you don't feel guilty since you paid a fair usage fee
- your friends get to keep the furniture afterwards since they still own it
option split items
Reubens4Dinner also has a good idea to split costs differently. If everyone brings in some things they can afford and everyone uses then nobody will feel left out.
They get a say in which nice things they want without being limited by cheap prices. You compromise by buying a more expensive mug set than you would normally but can afford since you don't have to pay at all for a dish set for example.
With such an arangement everyone can be happy since you all get to use the nicer stuff and nobody complains about unfair since the Ben Franklin effect kicks in (more elaborate explanation in Reubens answer, not going to repeat it all).
option accept gifts
This one is a really personal thing on your end which may or may not work once you realise what your friendship is worth to them. Consider their perspective. They are able to afford the more expensive stuff even without you sharing the cost. They enjoy living together with you. The only way for them to have both is if they pay for the expensive stuff and tell you not to mention it.
From their perspective you can make them happy by just accepting it and never mentioning it. That way they get everything they want so it's a win-win right?
The only issue here is you feeling like a freeloader. Since this is entirely on you it depends on you whether or not you can get over this. They certainly don't feel like you're abusing their generosity in this case (I know I wouldn't) so all you got to do is talk things through and make sure this is indeed how they feel about it. It really could be that easy (but my friend made me realise this won't work for most people).
What would I do?
What I would do in your situation is sit down with all 3 friends and explain what is important to you. That you can't afford the more expensive stuff and just accepting them to pay for everything makes you feel like a freeloader.
Then listen to what they consider important. Don't decide what they'll think for yourself, be ready to actually let them explain it.
It's possible they don't mind the money at all and just want all of you to be happy together with nice stuff. It's possible they finally realise why you are nagging about cheap stuff all the time instead of chipping in on what they think are trivial things. Most likely it's somewhere in between.
Once everyone has pointed out what they consider important you can propose the above options and try to work out what everyone likes. I would probably suggest a combination of the options. Like the usage fee one for furniture, the split cost for general not too expensive items (dish sets, bottle opener, toilet paper, ...) and accepting gifts for things like decoration.
As long as you all agree on how to split the costs based on what everyone can afford everyone should be happy and you don't need to feel bad about paying less knowing they would rather pay more and live together using the nice stuff than giving up on one of those 2 things.