So I've been allowed to get the tank soon. What would I need to get in supplies, eg. food, siphon, water test kit, etc. Could you also recommend some brands. Thanks

Oh, it's prolly gonna be an AquaPro 620T tank. About 130L. How many goldies could I have 3 goldies in there, or should I just stick to 2?

you can prolly keep three goldfish in there.. sounds like a 34 gal tank.

It's 130L, so around 35 gallons.

The best foods IMO are Hikari, Omega One, Aqueon and HBH.  Also, the frozen foods are great and don't forget the fresh veggies!
Where are you from and what stores are available to you?
What size tank are you getting?
If your tank is over 30 gallons I recommend a canister filter and maybe a power filter, internal or external.

Also, should I get some aquarium salt. I'm planning to get orandas and I've heard their wen can get infected easily.

Any particular brand you would recommend? Also, would stuff like stability work? And finally, should I keep an emergency supply of melafix/fin rot medicine/ the one starting with "P"?

Do you think I could have 3 orandas in a 130L tank?

40 liters per goldfish, so 130 will support three goldies
I would really suggest an external canister filter, or the biggest internal thing you can get! The filter should turn between 5-10 times the tank volume per hour, so aim for something around 650-1300L/h
And air pump + line + stone would be good to have too! Extra aeration is great to have in case you'll have warm summers Goldfish need all the oxygen they can get
Buy a siphon too and a few buckets, siphons are cheap! it's like a bottle at the end of a tube pretty much, and it's great to have when changing the water! Test kits and the like have already been mentioned....
Get some good quality food for them fish too, I'd like to add JBL to the list of good brands The pellets get soft quick so they'll be easy for your fish to eat, and you don't have to soak them for too long before feeding. Stock up on some frozen food too, like blood worms or something Growing fish need lots of protein!

I swear by live plants and natural environments for your fish, so I'd go right now and get some gravel, bog wood and pebbles But that's your own choice Gravel will mean more space for the beneficial bacteria to grow!

Do you reckon I could put 2 plecos (or whatever those fish that are small and eat algae) in the tank with 3 orandas? And, do you think I should get some red cherry shrimp, or will the goldfish eat them too quickly?

In this case, I wouldn't add the plecos, because most species grow REALLY BIG, and you'll overstock your tank. Especially since this is your first tank (?), it's not to recommend.
There are only some plecos that will get the habit of sucking on other fishes' slime coat, for example some of the gibbiceps-species etc. BUT, you can "easily" avoid this by keeping the plecos well-fed on vegetables and live foods There are special "staple" pleco pellets available as well containing algae or shrimp that you can buy. But the reason why I said "easily" is because it's pretty hard for you to feed the plecos with three hungry orandas around! I'm having a bit of trouble, so whenever I feed the pleco, I have to make sure that the goldfish are busy with something too xD But so far, they're all doing good And they just LOVE zucchini

The point: Avoid the plecos for now, you can get them later if you'll get another tank or a different setup They'll do better with some smaller tropical fish or something so that they'll get food easier

Yes, they'd be good Actually, it's really easy to keep a tank with a mix of fancy goldfish, since most of them have the same "disabilities". Orandas go with most fancy goldfish, like fantails, ryukins, ranchus, lionheads, pom poms... I actually keep an oranda with all these and some black moors, but you may want to wait with the moors until you have some more experience with the goldfish and how much food they need! Since moors are pretty slow due to their eye sight, the orandas might steal the food from them.