We have 2 neutered male cats both of them just over a year

Congratulations on the new arrival of Mabel! Unfortunately, introducing a new kitten can be a minefield and quite often the 'settled' cats can find this change very stressful.
Consider taking a step back and re-introducing the kitten more slowly, especially if it was initially done fast. This method works well for most: https://icatcare.org/advice/introducing-a-kitten-to-your-cat/
The problem of urine marking you bring up is a very common one and we tend to see it in multi cat households more than in single cat homes as mixing cats is a naturally stressful event. This is because cats find it tricky to co-exist at times and we tend to see evidence of this in their litter habits!
For the male, it may be that he does not want to use a tray a new cat has used or he feels he needs to let her know that the territory is his. It is also possible that the spraying is just a result of general stress. Cats show subtle signs of being stressed and it can be hard to know what is going on inside their heads. Certainly, one of the number one causes of feline stress is living with another cat (even if they seem to get on, are related or have always lived together).
We first want to use an enzymatic cleaner to clean anywhere inappropriate he has sprayed, to prevent him re-marking on it.
If he is not yet neutered, now is the time as this often dramatically reduces inappropriate urination/spraying.
Ensure the environment is as low stress as possible.
For a 3 cat household we should have 1.5 trays/cat, which is 5 trays! Sounds a lot I know, but any less tends to result in issues. Trays should be kept meticulously clean. They should be spread out around the home (some high, some low) in areas or low foot traffic. Consider trying different litters as cats can have preferences.
All cats should have their own resources, space, hiding places etc.
Continue using the plug-in which, over time, may help with harmony.
If you notice any increase in the frequency of his urination, straining, increased thirst etc. it may be that he has developed a cystitis/FLUTD, urinary crystals or an infection. For some, the only sign is urinating outside of the tray. These conditions can be precipitated by a stressful event.
If the problem continues, have him checked over and his urine analysed for any abnormalities. This website has some really useful advice: https://icatcare.org/advice/feline-idiopathic-cystitis-fic/
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No worries at all.
If he is a nervous boy, you could also consider the use of some natural calming supplements in his food for a few weeks while the kitten settles in. Zylkene , Nutrcalm and Yucalm are all good options.
Absolutely we would hope that his behaviour will gradually improve as they all establish their places within the home and learn their boundaries. Fingers crossed this won't take too much longer.
Certainly, spaying her will reduce any hormonal behaviour she may be showing and may improve relations. Spaying young (5 to 6 months) also dramatically reduces the risk of mammary cancer and uterus infections so is a good move to make health-wise as well.