Trainer is an idiot.
However this will be a lot of work. Might want to look for a specialised behaviourist that is close to you. This is how we're working on it, aided by a trainer:
Teach your dog the watch me command. Best thing is making a noise with your tongue, clicking noise, if your dog looks at you treat. If you use clicker, then click, then treat. Start doing that every few hours for a few minutes. Tongue noise, or 'watch me' or whatever you choose, the dog looks at you, treat.
Then start adding distractions. Dog is playing, you make noise, dog turns to you, treat. If he doesn't turn the distraction was too distracting and you need to take a step back and maybe try with less distraction. (For some dogs just sniffing is already a HUGE distraction and they can't be interrupted at first).
Once the dog looks at you even when playing you can start outside.
Find out how close the dog must be to the trigger for him to react. Then make sure he never gets this close to the trigger (in your case other dogs).
Dog approaches, try and get out of the way, safe distance, keep your dog's attention on you with your 'watch me' command or with tongue noise or whatever you decided to pick. If he reacts you were too close. If he doesn't, big reward. Now this every day a bit closer. There will be many setbacks. But eventually the dog will associate other dogs or humans with you having lots of tasty treats and he will focus on you as soon as something comes by.
With a 3 year old that learned this behaviour for several years probably it might take months...
My dog is leash reactive and he's 6 and we've been at it for almost all of those 6 years and he was perfectly socialised. He's just very anxious. We still have setbacks but 98% of the time his attention is on me when a trigger walks past.
Btw, maybe your dog doesn't like treats but then maybe he has a favourite toy? A frisbee or a tug of war toy?
Then train that as a reward for him looking at you, you throw his toy or play a short 20 second tug of war game. Outside as well, trigger walked past... yay, frisbee.
If normal food or toys don't do it, try high value food, peanut butter, chicken, cheese, if he goes nuts for it you can have really tiny bits of cheese that last you the whole walk.