3.4 is the exact age gap i have with my 2.
i went with the scate attachments. we had a bugaboo cameleon so got the bugaboo scate.
we also had a steelcraft fold up stroller so got a lascal mini buggy board for that from toys r us.
worked perfectly. DD1 could stand on the scateboard if she wanted or i could put it up if she wanted to walk.
i found over time she used the board less and less and now just over a year on i hardly get it out of the boot anymore. it fitted in nicely when we did need it to bridge the gap