I think probably if he's having 3 lots of 3.5-4h A time in the day (and genuinely needing that long after a short pm CN to settle) then the day is likely too long and he is getting OT from too long a day and too short a night. That only allows for 12h total sleep in 24 which for most LOs just isn't enough at this age. It's a messy thing the 2-1, you sound like you're well into it if that's the case (we also started some one nap days from 10 months).
Things I would try:
Rule out OT first - seeming to need 3.5-4h after a 30 min CN could well be a very OT second wind. I would try something like 3h30-45 first A, 'long nap', 3h45 second A, 30 min nap, max 2-2.5h A before bed
Keep day to a max 12.5-13h (13h was always too much for DD and usually still is)
If no joy after trying consistently for a few days then you may need to take the plunge and start trying some one nap days. Or cut your pm nap to 15 mins only and get him into bed earlier. I just think at the moment he has too much total A time and you need to find some way of reducing that. What do you think?