Doing a first marathon without cutting the swimming and biking

Ha!  good luck.  With marathon training, you'll be pushing your body to the limits with running.  Throw in any (serious) cycling on top of that, and something has got to give.  However, light cycling can easily be continued as cross training 1 or 2 days a week and will contribute to your aerobic base.  However, if you're thinking of strenuous intervals, hill work or long rides- they'll leave you fatigued for your next run.  it will take its toll.

Yes- you can do both.  It will be sub-optimal.  I did run focused tri training this summer/fall and it did worlds for my run pace.  My bike didn't get any better, but I was 30s/mi faster in my annual half mary a couple weeks ago.  So, pick your priorities.  I'd suggest no more than 2 days cycling/wk, with one easy recovery ride that's perhaps 1 hr, and another 2hr zone 1/2 ride on sunday, after your long run on saturday.  Always put the long run the day prior to a long ride if you need to be run focussed.

You could bike 3 days a week with a run either right after or later in the day.

Run 3 other days.

And swim on the 7th day.  This would be an easy workout.  No hard kicking sets. 

Sorry, but i have to disagree - it won't necessarily be sub-optimal.  I ran my best marathon the year I cut back on run mileage and kept up the swimming and cycling.  All my running buddies told me I was crazy, but the PR told a different story.  My biggest week was probably 35-40 miles of running, average was more like 20 miles a week with 60 miles on the bike and 1-2 days swimming - perfect recovery after a long run.  

Few junk run days, more quality and good recovery.  Good luck.  I hope you find a good balance and rock the marathon!

Depending on the bike/swim mileage, it just sounds to me like IM training.

Use the Hal Higdon Novice 1 marathon plan.  4 days/week running.  I swim on the same midweek days as I run often.  With no ill effect on either.  Then split the weekend with biking on one day and running on the other, probably best to run first then bike the next day so you're not running on such tired legs and at a greater risk of injury.  I'm no certified coach or anything, that's just what I might do.

I did my first and second marathon this year.

For my training for them I ran 3x per week getting up to a max of 3hrs 20 mins for my long weekend runs then I did another med/long run during the week which maxed out at 1hr 30 and then did a 1hr run during the week which mixed in lots of hills and a few intervals.

I continued training 2x gyms, 3x bikes and 2x swim per week on top of that too. They were all relatively easy and never more than 1hr 30 mins. 

So it is possible for almost anyone. I used to think running a marathon and doing the training was daunting, but in reality, it wasn't much different from my regular tri training apart from my weekend run was a lot longer.  

I did a 16 week FIRST (Furman Institute of Running) plan that's based on 3 runs per week and it worked pretty well.  It's pretty low volume for a marathon plan but focuses on one speed workout (.25 to 1 mile repeats), one tempo workout (3-10 miles) and one long run (10-20 miles) each week.  Paces for each are calculated based on current 10K pace. 

Part of the plan is crosstraining a couple of days per week, but if it were me I'd focus first on getting in quality run workouts and dial back the intensity/time/frequency of swim and bike training as needed to not impact the quality of the run workouts.  You'll probably be sick of running by March so that will be a good time to ramp up swimming and biking again.

All aerobic training helps.  sport specific training helps more.  The great thing about cycling is that you can put huge gobs of time in zone 2/3 and it has less recovery time than running.  What you proved was that doing a fair amount of running and adding cycling and swimming to that allowed you to improve.  of course, you'll never know how you could have done executing a more marathon specific training. And- I guess I'd agree, it wouldn't necessarily be sub-optimal, depending on how many miles your body lets you run.  If running injuries limit your running mileage but you can still cycle, then it could be more 'optimal'.  Each person is different and each person's marathon goal is different.  Not too many runners are BQing while peaking at 35-40mpw and cycling 60.  But run 60mpw and no cycling, and that can get you there.