It looks like the 'I want to move up in the pecking order' behaviour shown by my girls towards one another. But your two larger piggies are being very very patient, normally there would be a really pointed sideways head swing buffet by now and the littlest would give up. I don't know whether they are being patient just because they care as she is smaller than them so babyish in their eyes or because it is a boy.
The attempts at moving up the pecking order will occur sporadically for quite a while if it is that - in my experience. You'll know that Treacle has accepted her position when she begins to follow them around, nose to bottom, chuckling to let them know she's there. That's a display of 'I'm lower than you' behaviour. She'll also try to take what they're eating from their mouths which is baby behaviour, and push her face into their sides. If it's a boy - then I don't know how this will develop as I have never kept boars!
Hope this helps, I can only say about what happens in my little herd.
Sarah x