HIM 3.5 months before IM

Sort of two different things at play here.  3.5 months out is still beneficial.

No way would I do a 9-month training period.  There's a high risk of burnout doing that.  That's arguably more mental than physical though.  I have done that once or twice where I trained for almost 10 months straight and I was absolutely fried by the end of it.

If you want to do both of those races I'd suggest maybe doing a 5.5 to 6 month training cycle where you trained the first 2 to 2.5 months for the HIM and switch over to IM training for the last 3.5 months.  You can be very well trained for a HIM on 8-10 weeks of training.  Take a week or two easy after the HIM and then do a solid 12 week IM training cycle.

Do not think of it as 9 months of IM training.  Train for the HIM and train to do it well.  You don't need to start a HIM plan today.  But if you haven't been training all 3 sports regularly, then you should begin getting into a 3 sport routine over the next 2-3 months (if, for you, that does mean following a plan, then so be it).  Take a full recovery after the HIM (maybe 2 weeks or so of easier/unstructured training) and then begin training for the IM (if the plan you choose is longer, simply use the last x weeks to match what you have left until your race.  If you've trained well for the HIM, 10-12 weeks is plenty of "IM-specific" training.