Firstly, hello

I'm new here. I've posted this in Cat Chat too but figured this is probably the best place!
I'm Kayleigh from South Wales and I have three fur babies. Dexter and Oscar who are around 1 and a half years old, and Lola who is around 5/6months old. I'm a veterinary nurse and adopted them all through my work. Dexter was the first, then when he was around 12 weeks old we got Oscar (also 12 weeks) They get on relatively well, but mainly just get on with their own things.
Then there's Lola, she got dumped at my work too. I couldn't resist, a beautiful little silver tabby. I didn't think it'd be a problem. Oh boy was I wrong. I researched how to introduce, (although I didn't do this with Oscar as they were both so young - I guess I thought if I ever got a third, I'd just let them fight it out again and all would be fine!) So she was in a crate quite a lot or kept upstairs while the boys were downstairs and visa versa. I did scent swapping and gentle introduction but Oscar would just growl at the sight of her. Dexter was the opposite, very curious about her - and after about 2 days of hissing from him, they were rolling around together playing and grooming! I couldn't keep them apart, he would run and sneak if a opened a door. 3 months on, they're still doing ok. He does spend more time outdoors to get away from her (she is annoying!) but tolerates her well. After two weeks of keeping them separate we decided to do a full intro as the growling had stopped. She just wants to play, but Oscar is terrified of her. He's got no problem warning her if she gets too close. He spends a lot of time hissing and sleeping in his high top bed where she can bother him, but has learned not to. The problem is that 3 months on they're no closer to getting along. Occasionally I find them play fighting... well its hard to tell as I think Oscar would like to kill her

They bat at each other and chase, but if she gets too close he backs off - he's still frightened. In the last two weeks we've gone backwards a little in progression. They were all sleeping downstairs together, however Oscar has begun urinating behind my curtains. I can only imagine she corners him there and he's too scared. Its not scent marking as they are full puddles and I'm sure its Oscar as Dexter has no problem using the tray. (They are outdoor cats, so do their business elsewhere normally, but they are kept indoors at night time) I have three litter trays currently, and I know they advise one per cat and one extra but I don't know if I have anywhere else to put a forth? And is it still needed when the boys are predominantly outdoors when toileting? Last night we bought Oscar upstairs to sleep with us, he's a very independent cat but loves sleeping on our bed with us. We bought a bed, his bowls, and the litter tray that lives upstairs anyway and he slept peacefully. Shall we do that so he feels safe for a while? Is there any way I can make them get on? Its funny because Oscar regularly brings random strays home with him, and outside the house is very friendly (towards people and other cats.. even dogs!). He's even turned on Dexter somewhat, I catch them scrapping more and he hisses at him. I suppose Lola has questioned his dominance and I do believe she may end up above him in the pecking order when shes bigger. I forgot to mention that the boys have been castrated and Lola was spayed last month. I have two feliway plug in's that are constantly on. Oscar has no problem eating, but is intimidated by her when walking around as she is a typical kitten and follows anything that moves. At first I was annoyed that he hadn't taken to her - I mean, how can you be scared of something so tiny. But then I realised that his whole world has been turned upside down and now I just want to make it better for him.
Any suggestions? I'm sorry for the massive story but didn't want to miss anything out. I know they may never get along, but how long do I give it? I'm not frightened to rehome Lola if Oscar will never be happy - after all, he was here first.
Thank you in advance