16 weeks until HIM training

Why not schedule an intrmediary goal race for one or more of the sports. For instance, a half marathon and/or metric century on or about that 16-week mark. Ideally you are already confindent about those distances and can follow a more specific plan to reacha time goal...

would building base endurance in any or all of those sports not be a really good idea?

I have no experience with HIM but surely you would benefit by doing at least 1 long run or ride every week, thats ultimately at least twice what you're doing already?

i did my 1st HIM last year.  i would recommend on very slowly building endurance up.  just work on the endurance part.  and when you begin the training you can bring in the intensity factor to it.

i had a 12 week training program and although i followed it "pretty" close.  i didn't put in as much intensity into the program.  I would say doing that will give you more confidence come race time where you should be.  BTW 12 weeks seems short but i was on a 16 week olympic prior to that.

FYI i got off the bike, had a good average and hit my goal, but had to walk after mile market 9 due to insufficient run fitness.  what i have learned is that the longer distance really require you to have the proper endurance level and training at the right intensity.

in your case, if you are doing this workout 2-3 times a week.  on the weekend maybe do double on the bike, and work yourself into longer runs 60-70 min.  come training time you won't get shocked when your plan has 2 x 70 min runs 1 x 90. 

I just completed my first HIM last year.  I used the BT 20 week training program.  And while I couldn't complete every training session, it had me ready to finish my first HIM.  I used an Oly race half way thru to get a good judge on where I thought my fitness was.  Actually did a PR on the swimming & the bike.  So felt I was on target for the HIM..