Not sure about the train, but I do know it's a pretty easy drive to Lake Orta from Malpensa. And it's a lovely place, too.

That said, Lago Maggiore is very nearby as well. You really couldn't go wrong either way.

If one of the main convenience issues is train travel to and from the lakes, I would look at the train schedules to and from each of the lakes and make the decision on that basis.

Several years ago there was a bus direct from MXP to Como. The MXP website should tell you if it still runs.

we will not be renting a car - I was hoping a poster here would have experience to tell me which lake would be easiest, probably by train, and I could look up the details for the times. Thanks!

IMO it is easy to go to Lago Maggiore (to Stresa).
It depends on the month when you travel. Between end of March and end of September there is a direct bus (Alibus), very easy to travel with it. See http://stresasights.blogspot.co.il/2011/03/transportation-alibus-shuttle-from.html and http://www.safduemila.com/files/alibus-2012-english.pdf

In other months you have to take the train (you have to change it in Busto Arsizio), see http://stresasights.blogspot.co.il/2010/10/transportation-new-train-service-from.html

I was hoping a poster here would have experience to tell me which [my emphasis] lake would be easiest,

That assumes that the poster has used public transportation for the various options that exist; unlikely, in my opinion.

I agree that Stresa on Lago Maggiore is the most convenient---buy airport bus or by train from Milano Centrale.

If you want to go to Stresa from MXP, take a taxi or a bus to Gallarate station - it is a short hop - and then train from there.

I don't know if it's the "most convenient" way to go, but there is train service from Malpensa to Stresa via Gallarate. Some options take about 90 minutes. Just search from Malpensa Aeroporto to Stresa:

http://www.fsitaliane.com/homepage_en.html

The most convenient for me would be a shuttle service or driver from Malpensa to Bellagio or Varenna on Lake Como. You might look into Fly to Lake service (link below) to see if their timetable would work for you. A driver or taxi would be even more "convenient."

http://www.flytolake.com/home/index.php?option=com_content&task=view&id=15&Itemid=1

It is roughly an hour train ride from Milan (city) to Varenna on Lake Como.

The OP is starting from Malpensa, not central Milan.

I have been to Maggiore, Como, and Garda - all by public transportation. They are all possible without much difficulty from Malpensa. Even the furthest, Lake Garda is only a little over two hours. Yes you'd need to change in Milano Centrale but that is not hard to do.

So I would say pick which lake you would like to spend three days at, rather than which one might be a few minutes less travel. You say you are going on to Venice afterwards so even traveling to Lake Garda is not going to be out of your way.

Of the three, Maggiore was my least favorite. I would go back to either Como or Garda (and in fact I certainly hope I do).

Touring the area is much easier by car.

We started a trip with a relaxing 3 days at Stresa. The town is large enough to have a good selection of hotels and restaurants. There is wonderful ferry service to the various - and very different - island in the lake and there are several charming villas and gardens to be visited in the area (but not sure how without a car).

Lago di Garda is a bit farther, but it is right on the way tomVenice ( by train), so you would not be backtracking.