At every consignment sale I've been to lately, it seems there is a triple stroller (or sometimes more than one). You might check those, if you're on a budget. Otherwise, they're out there, but they're pricey. Peg makes one that I've seen in person, but Pegs are made of plastic and get bad reviews on lasting performance. The SBS ones I have seen look way too wide for doors, but the tandems don't look much longer than a Graco Duo. Good luck!
Re: Stroller ideas for 3 kids