First tri season done, training for a marathon, need guidance

Are you planning on doing more tris next year after the marathon?

I would personally add in a 4th run, but you could certainly "just finish" with 3 per week. My "training-for-a-run-but-still-cross-training" schedule looks like this, just as a reference. I've found that running back to back on the weekends really helps my legs figure out how to run "tired." Example distances for back to back weekend runs for me in my most recent training plan were 8/3, 9/4, 10/rest, 9/6, 10/8, 8/10, 13.1/rest, 7/5

Monday: Bike (1 hour to 1.5 hour)
Tuesday: Short/tempo run (3-5 miles)
Wednesday: Bike 1 hour (you could replace with a swim here)
Thursday: Medium run (4-8 miles)
Friday: Rest
Saturday: Long run #1
Sunday: Long run #2 or medium-long run

Have you checked out marathon training plans?  How does what you propose align with those?  

Everyone is different.  Most of the runners training for a marathon have the long run be no more than 30% of their weekly volume.  Your plan will have it almost 60%.  They'll also consider 35 mpw quite low, but that amount is consistent with IM training plans that include the marathon.  

Do you really like the tempo run?  They increase the likelihood of injury which runs counter to your "just finish" goal and for the 3-runs-per-week won't deliver as much gain as they would in a 5-runs-per-week regime.  

As for "maintaining" bike/swim fitness.  Are you doing those just once per week already?  If so, then a once a week workout will be enough to "maintain" where you are.  If your HIM plan had you doing each 2-3 times per week, sorry...you can't maintain a level of fitness cutting 1/2 or 2/3 of your workouts.  

Have you looked at the FIRST marathon plan?  LINK

It is a three run day plan with two cross training days that you use for swimming and cycling.  It would work well for you.  

If you're a swimmer, you could always just knock out 1500y or so, and then do your short run.  Not sure if your schedule allows you to double up workouts.  Honestly, you could probably even drop the swim if you swam back in school.