I have 2 kittens and could use some advice?

They're still learning. Watch them closely (Look for signs of when they're gonna go to the bathroom) and when you notice them about to go, pick them up and put them in the litter box. They should eventually catch on that they pee there too. They may not correspond with that quite yet...

They dont want to share the litter box. Every cat should have its own litter tray and, ideally, you should have one extra. Therefore, with 3 cats you should have 4 litter trays. The size of it does not matter, they simply dont like sharing. What happens is that the litter gets wet and then when they have to get back into the box to poo, the wet litter gets stuck on their fur and they have to lick it off - not nice for a cat. This is particularly true if you have long haired cats. You should also make sure that the litter is as soft as possible on their paws - it should be as close to sand as possible. Never use scented litter as, again, when it gets wet they have to lick their paws and injest the scent.

One other problem could be that your older cat is "guarding" the litter tray in an attempt to be territorial. It is easier for a cat to hold on to the contents of its bowels and go when the coast is clear, but a full bladder has to be dealt with far more quickly and if another cat is stopping them from using the tray, they will have to go wherever they can. If you think this could be the case then you should distribute the extra litter trays around the house as it is impossible to guard all of them at the same time.

Try 2 boxes. How old are the kittens (they should be neutered by 3-4 months old especially since they are opposite sex)? Can the kittens get into the box easy?

Maybe try a second box with a different litter.

Try to put some of their pee in the litter box so it smells like that's where they're supposed to go. It also might help if you put them in the litter box if you see them anywhere near it.