I had twins when my son was 19 months, I should have invested in a triple stroller b/c it was quite a task to do ANYthing...
I did get a side-by-side and a reg. double stroller... The double one was a Graco and it was nice b/c you just snapped the twins infant carriers right into place. Once they outgrew their carriers I invested $ into the side by side... I got a Combi and it was over $300 but it is a very nice stroller and a little more compact that others.
My guys will be 4 soon and the stroller is still like new. No problems. Actually they are each approaching 40 lbs and that is max capacity for the Combi side-by-side so I think I will be parting with it soon.
I haven't used it in months anyway...
When all 3 were tiny, I'd usually put the 19 month old in the back of the Graco stroller, one twin up front and carry the other twin in my front carrier... This is how I grocery shopped etc...
You're child is a little older so you might want to wait and see how things are once time gets closer - if you can avoid the double... They are a PITA in some stores...
The side-by-sides are supposed to fit in ANY plane that is handicap accessible but that is not always the case.