Scott McCloud's Understanding Comics has a whole chapter on the passing of time in comics, you should check it out!
It's always better to show things are happening rather than telling a reader something is happening, so showing that time is passing without a "three years later" text box is an interesting challenge, but worth the effort.
But if you want an example, here's one from my favourite webcomic, Johnny Wander by Yuko Ota.
The passing of time is illustrated by the branches in the corners changing, as @jessiepolfliet suggested. But there's also a LOT of panels, showing a lot of things are happening as time passes!