Lots of variables here - Doing the HIM as a training day or racing it? Expectations for IM - survive or go hard?
Especially with a first IM - I would not risk adversely affecting my IM with a HIM only 3 weeks out.
A HIM is also not a real good indicator for an IM. Nutrition and pacing strategies are very different.
Tho - you will probably get lots of varying responses - but you need to judge how well you can recover from the HIM and still have a good taper.

I've always done a HIM before my IM's three weeks out...great start to a taper. Mind you ,though I didn't race them. Just used them to work out kinks, double check (ok quadruple or quintuple check ) my nutrition (taste and delivery method, not quantity)...Sort of a shake down cruise. The distances are a little short for the last long workouts, so I always went out the next day for a long ride and a longish run off the bike. It also helps get your mind race ready
Good luck!

IMO this question needs to be answered before the decision can be made, with the follow up question of: How did you recover from the HIM?

My point being that recovery is an individual thing. I do not recover well from racing 1/2 mary's and full mary's but I do seem to recover very quickly from racing OLY's. After racing a HIM 2 years ago I was able to run a fast 1/2 mary and full mary a week and 5 weeks later.

I did not have much of an energy level after completing an IM, so doing a HIM a few weeks later would have been close to impossible for me. Doing the reverse would have been very doable.

For others, all the above may be very different.

If it were me, I would avoid it, but if you were to do it, you will have to be very disciplined and avoid the temptation to "race" the half.

I think he's saying compared to the ironman distance a half doesn't stack up, which is true. However, I do see it to also be borderline offensive for all of us who do short course tri's. I have only completed 2 1/2's as my longest events and they were impressive to me.

3 weeks before an IM is when I am doing a Race Rehearsal weekend (read my article here about them: http://www.trifuel.com/training/endurance-files/how-fast-can-you-go-in-a...)

You will get far more benefit from an RR weekend than you will from the HIM. Although I tend to ride 112 miles and then run 6, however long it takes. That will tell me if my wattage choice is correct.

IMHO.

YMMV

+1 A RR is time better spent for prepping for the IM.

Why risk it? Four weeks I think is doable as long as your disciplined and can just treat the half like a long training day, but three weeks might just be pushing it.