Rabbit gone humpy - Trio Bonding

Okay, so this forums is about 2 main situations.

I have a bonded pair of rabbits, Otep (female, spayed) and Benji (male, neutered), but have recently decided to add a third bun to the situation. Basically Otep seems a little more independent than Benji would like.
I've chosen a beautiful boy, called Fiver, who is cocky and wants to play with all rabbits he sees, so I figured this would be relatively easy (as my original bonding was).
Currently, as I have brought them in for winter, these three share the same play space but at differing times. It is also how I wanted to start the bonding process to get them used to the new smells.
So first situations, any tips on how to further proceed? He's not neutered and so I'm holding back on more full on introductions until after that's been a thing.

Other thing, he's a horny little boy (roughly 4 months old) and I think his behaviour is influencing my Benji, who has started humping things (and when locked in the cage has started trying to hump Otep and making that bumbling noise. She just runs away so I'm not overly concerned else I'd separate them.
Is there anyway to stop him being like this towards her? Or will this just fade as it fades with Fiver.

If all goes wrong, I'm keeping all rabbits but separately, so please don't feel concern to any being homeless.
Happy for any corrections questions and suggestions.
Also, I'm a student so spend a lot of my time in my room on my laptop (right next to their cages/playpen) so can watch their movements most of the time if there any behaviours that I should look out for.

Thanks all. Sorry if its a tl;dr situation

If you allow the new bunny to go up to the other rabbits' cage it will possibly upset them hence the male has started humping the female. It could also spark off a fight. Your new bunny is possibly old enough to be castrated if his testicles are on show and this will help to calm things down. Bonding the 3 should be carried out in a neutral area as your older male might attack the younger one, or the female could attack him. Have you enough space for 3 rabbits?

Thanks for the response =D

From what I had read on the internet (trustworthy place I know), letting them meet before the bonding through the cages was a form of pre-bonding that made real bonding easier. I'm guessing that the behaviour exhibited by my two is therefore a sign that this method won't work?

The vets I've gone to (a Vets for Pets in a Pets at Home) have said they won't castrate until 5 months (another month), so if they aren't willing to risk it, I'm not.
I will hold off and do the neutral space thing then. Thank you. =D
And yeah I have enough space, my only struggle is giving them completely separate play areas for the time being seeing as though I can't leave either group unsupervised (Benji, I'm looking at you here!)

There is usually always aggression through bars. The caged rabbits do not like other rabbits coming up to the bars. It is the same if your rabbits are outside in a run. Yes, I prefer to wait until a rabbit is a bit older before having them neutered.

There's not really been much between bars aggression (less than when I attempted the original pairing), but I will hold off anyway as I know this will be considerably more difficult a bond than the original anyway. Or should I attempt an introduction before neutering? Seems like a bad idea to me, but obviously you'll know more than I will.
Thanks for letting me pick your brain on this, really don't wanna mess it up as I'm already attached.

Definitely don't attempt an introduction before neutering as you might have a fight they'll never forget on your hands. Rabbits seem very sensitive to hormones. When I brought Stakki home he was eight weeks old and we'd been told he was a girl! Our older 5 year old male, Niney, knew he was male straight away and started humping Squirrel (our 8 year old female) to display dominance. Both were neutered and hadn't humped for years. Niney hated Stakki and never mellowed towards him. If he had been female, I think they would have bonded after Squirrel's death but it was not to be.